7 numbers you should know about your car

Kyle Smith

Cars are utility objects first. They serve to allow us easier movement over great distances. The inherent mechanical intricacy and unique experience of driving caused users to morph into enthusiasts. Those folks dedicated to the joy of driving and maintaining these automobiles can obsess over so many small parts and details of their machines. Even the casual enthusiast knows a few little details of their car that they enjoy conversing about.

Those details can be quite banal, or very much intriguing. It really depends on how deep your knowledge of the automobile and its history is. To cover the basics:

Minimum octane rating

Parlanti Sunbeam tiger fill up
Kyle Smith

Engines are surprisingly durable, but also require particular care and feeding. The minimum octane rating refers to the proper fuel that the engine is tuned to run on. Most fuel stations have options for 87, 89, 91, or 93 octane, with each number referring to the fuel blends resistance to detonation as it is compressed in the combustion chamber of an engine. The higher the better right?

Wrong. Engines are tuned for certain octane ratings and putting high octane in something that does not need it will not give you more power or a cleaner engine. In fact, it’s possible the engine will run worse, and it will certainly cost you more at the pump. There is a balance though. If you put lower octane fuel than required it will certainly yield less power and can even lead to damage as the fuel can ignite before the spark plug fires, which can damage the piston, connecting rod, or cylinder head as the still-rising piston attempts to compress the expanding fuel and air mixture. Not good. We go through the effort to use the proper oil for the engine, so make sure you are using the proper fuel too.

Fuel range (ballpark)

On a long enough timeline everything fails, including your gas gauge or sending unit. When the needle stays at full or just drops like a stone to zero, do you know you can make it home? Knowing the ballpark range your car can travel on a full tank of gas is a good thing to know if you drive your vintage car regularly. Having non-working gauges is embarrassing, but running out of gas is even more so.

Start by doing a quick miles-per-gallon calculation by filling your fuel tank, driving for a weekend, then topping off and dividing the miles you covered by the number of gallons required to re-fill the tank. That can be the basis of calculating range using the fuel tank size and ballpark MPG. Be sure to leave some wiggle room in the final fuel range for conditions to change.


1927 Buick on dyno
Kyle Smith

Opening the hood of a car is often like rubbing a genie lamp: Someone will always appear, but instead of giving you wishes they will pepper you with questions. The most popular one you should expect to hear? “How much power does this thing make?”

It can be both an easy and a hard question to answer. You could go by the original rating from promotional materials, or look for similar builds that have been tested on a dynamometer, or if your engine is unique enough it would probably be fun to strap it down on the rollers and find out the exact power, along with just how well tuned the package is.

Ballpark value

We know you aren’t planning on selling your car. Money talks sometimes though, and your author has experienced at least one situation where a person approached while out and about and made an offer to buy the motorcycle that got me there. It was a real offer and if I hadn’t known what the bike was actually worth I probably would have taken it. Values change over time and can sometimes move quite quickly. Be sure to know what you are driving and treat it appropriately. You might have bought it for $1500, but that was 1988 and things are different now. Luckily, finding your car’s value has never been easier.

Vehicle identification number

Model A VIN on engine
Kyle Smith

It sounds funny, but having the main identifying information for your car is important. Not only in case of theft, but also in case you need to find information about the history of your baby. This one doesn’t need to live in your head, but keep an old insurance ID card in a file at home even if you sell the car. People go looking for the car from their past all the time and with the VIN you stand a chance, without that bit of information it can be close to impossible to find a car from 20 years ago.

Generation years of your model

Corvair early models at Orphan car show
Kyle Smith

The total production run is a fun fact that is easy to pepper into a conversation about a car that can often elevate someone’s perception of you nearly instantly. People will often walk up and make a guess as to the year of a car and in our experience, it can sometimes be a little rough. “Is that a ’64?” “No, it’s a 1960, but the body is the same so it’s tough to tell” is a lot nicer way to tell someone they are wrong and start a conversation on the right foot than just saying “no.” Total production is a fun one to add to this conversation if you happen to have a steel trap memory and can keep track of something like that.

Normal oil pressure

Brandan Gillogly

Just like people, every engine is unique and that includes how much oil pressure it makes while running. Oil is the lifeblood of an engine and knowing that there is the proper amount circulating is peace of mind worth having. We admit most cars don’t have an oil pressure gauge with hard and fast numbers, but installing one is never a bad idea as it can alert you to something being wrong in your engine before it turns into catastrophe. Keep an eye on those gauges and shut the engine down quickly if something looks wrong. Better safe than sorry.

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    If you go to the trouble (and expense) to put your car on a dyno, you should also endeavor to know the torque numbers in addition to horsepower. In many cases, torque is far more important than horsepower. Getting my 3600 pound car moving from a full stop takes a large torque number, and I do that multiple times every time I drive. Getting to a top trap speed requires horsepower, but I may bever even do that again in my lifetime. When someone asks me “how much power does that thing make?”, I generally steer the conversation to the torque specs, which are much more impressive when one understands why.

    Very True! Pre-war car’s engines were designed around torque, not horsepower, and the torque number us usually bigger then the horsepower number, sometimes by a lot!

    Too many can not even explain the difference on gross HP and Torque vs Net Hp and Torque.

    These numbers mean little unless you in serious racing and you are using ever singe HP or FT LBS.

    It’s rather simplistic—and wrong—to think of torque and horsepower as independent properties.
    Horsepower is the rate of doing work. Mathematically, HP = T x RPM/5252.
    More horsepower gets your car moving faster. A lower torque engine will have to turn faster to make the same horsepower, but equal horsepower will cause equal acceleration.

    Every time I’ve seen an engine run on a dyno they include the torque. It’s part of the procedure. Unless you enjoy sending people to school about torque leave it out of the conversation. Most people at Car Shows want to hear the HP numbers and don’t really care about the rest 🙂

    You don’t need to know your horsepower rating. It’s one of those things that drive people to spend stupid money on car projects. If it produces the appropriate amount of tire smoke when requested, and sets you back in your seat the right amount when you stomp on it, who cares if you didn’t actually break 300 horses

    Oil pressure – one of the reasons the gauge has largely been replaced by the idiot light because generally speaking there is no ‘right’ number. It will go up to some degrees with RPMs and down to some degree with oil temperature, and oil temperature can vary considerably based on ambient conditions and how hard you are pushing the car. I also would not make the assumption that the oil gauge in your dash is a highly calibrated precision instrument

    You are exactly right about oil pressure gauges not being precision instruments and having fluctuations during use. On the other hand though is the fact that the vast majority of them are consistent. I prefer consistency to accuracy every time. The article is more hoping to encourage folks to look for where their engine’s oil pressure normally reads and if it suddenly is something different, to go through effort to figure out why.

    Correct – consistency is paramount. And – know your baseline so a change is apparent, even if not in a bad zone yet.

    That’s why race cars have the gauges rotated so the needles point straight when ‘good’. Specific values are less important and a quick glance shows all good.

    You’re missing the point regarding oil pressure.

    It’s not about absolute values (though, I will argue that every vehicle I own *has* been hooked to a well calibrated AutoMeter mechanical gauge for at least a little bit), it’s about understanding ranges. Hot my motor is normally about here; cold it’s about there, so… Normal operating range is between those two points, roughly.

    Knowing that on an engine literally saved my life (or at least pain, and a pricey rebuild), flying a PA28-181, I noticed a dip, and turned back to the field before the Lycoming seized. Also saved a W-motor put together by an “expert” rebuilder who didn’t do a good job putting a pickup on a GM oil pump. Dip in cold oil pressure raised alarms, and some diagnosis.

    Learn your mechanical devices. A little familiarity can make your mechanical buddy not kill you, or at least last a little longer.

    Looking at gauges at a glance and knowing whether things are right, not necessarily the exact number. That’s why professional cars have the gauges mounted so that all the needles are vertical when the ‘number’ is correct. A quick glance will tell you all you need to know.

    Yes! This! Indexing the gauges also shortens the “eyes off the road” time, increasing safety. Of course, explaining this at car meets to folks who ask: “What’s wrong with your gauges?” is fun as well.

    When I was a young gearhead back in the 70s and early 80s, a buddy who raced a 67 Camaro (10.30 @ 130mph!) took me under his wing and taught me a lot about everything from staging to shifting. One of his lessons was that you didn’t have time to interpret gauges, so point the ‘normal’ straight up. Same for the shift point on the tach.

    As I near retirement, I am starting to get back into my old hobby, and my 78 Dodge truck and my 85 Monte Carlo gauges are all set up that way. Now I just need to find a young buck to pass along the ‘old ways’, just like Kenny passed down to me 50 years ago.

    All young guys need an “Old Man”…not a father, but an older guy who takes the time and trouble to introduce interested young people to things mechanical. Answer the “stupid questions” that cause most Dads to roll their eyes.

    In my younger days, I raced a VW sedan off-road and on dirt tracks. It didn’t take long for me to realize that I didn’t have time to look at gauges, so I rigged up a set of trailer clearance lights to tell me if oil temperature, oil pressure, or revs were out of range. When something went wrong, I knew it immediately, no checking glance required.

    On a date with my (ex)wife – her in a short dress and high heels, I noticed a dip in oil pressure on my E Type. Pulled off at the next exit to find an oil line had burst and the oil was dumping. Had I kept going for our date would have probably ended up with a written off engine. Of course, she loved riding in the tow truck driven by Clem in her little club outfit…

    I remember back in the day when I owned a 1962 Ford Cortina 1200cc that had an idiot light that never came on until the engine was switched off. Then my wife bought me a set of guages for oil pressure, water temp. and ammeter and from then on every time I stopped for any reason the oil pressure would drop to about 15 psi and I would bog my rods waiting for the bearing rumble. After a 2000 mile trip I couldn’t stand it any longer so I disconnected it and never had to worry about oil pressure again! Welcome back the idiot light.

    Back in my younger days somebody told me that it only took 7 psi to turn off an idiot light but pressure that low would ruin an engine. I put gauges in any car that did not have them.

    I agree, TG. It produces tire smoke and sets you back in the seat because of torque, not horsepower. So, I again reiterate that if there is a number to remember from your dyno test, it ought to be torque!

    Imaginary oil pressure gauge . . . when I worked for Max Hoffman, the BMW importer back in the late 60s, the 1600-2 and 2002 cars came with a fake oil pressure gauge. It was electric and didn’t measure anything. You turned the ignition on – and without starting the engine – you instantly had “oil pressure”.

    I asked why there wasn’t a REAL oil pressure gauge, they told me that the buyers were making the dealers nuts asking “why is my oil pressure X?” because they really had no idea of how the gauges worked or what the indications “should” be at varying speeds and temperatures.

    They felt an idiot light would be insulting to their enthusiast market (Remember, this is a “Driver’s Car!”) so they settled on an idiot gauge instead. In reality, the vast majority of drivers are perfectly OK with the light, which only tells them when something is not OK.

    I still prefer actual pressure gauges (and oil temp gauges as well) because it lets me monitor “trends” instead of only telling me “Your engine just blew up, call your dealer.”

    totally agree, what a place in automotive history to learn from. I would throw in Vasek Polak, Brumos, etc as well. great idea.

    There are many other vehicles that have similar “fake” gauges – some of the Ford trucks had those years ago (may still). As long as the oil pressure, for example, was above 9-10 psi, the “gauge” showed it to be OK with its needle position.

    I have a mechanical gauge in my Bronco and it’s amazing to watch the oil pressure fluctuate as the engine temperature changes, rpm changes, etc. I can see why it would drive consumers batty….

    You are correct, my 1994 F150 has an on/off switch which drives a “gauge”. If the switch senses 3lbs of pressure, turns on the gauge to read middle of the gauge oil pressure. My gauge was fluctuating so replaced….that is when I learned about the fancy idiot light…

    Those Bimmer gauges are just ammeters with an oil pressure face substituted. Used to see those on cheap used cars in the Midwest sometimes. Dead giveaway: turn the lights on and the indicated pressure drops!

    A friend has a replica Shelby Cobra with a 427. About once a year, someone at a car show asks what mileage the little car gets. I tell them that, if you drive it wrong, you get about 15 miles to the gallon, but if you drive it right, you get about 5. (I don’t know if my numbers are right, but they are close enough for anyone asking that question.)

    I know I will get some pushback on this, but once you have a horsepower curve for an engine, there is actually NO additional information provided by the torque curve. These two curves are just two different ways of saying the same thing. Horsepower equals torque times RPM. There is no way for the torque curve to change without a corresponding change in the horsepower curve. This doesn’t mean that a torque curve isn’t useful and interesting, but it’s not independent from horsepower.

    I know I’ll get some pushback on this, but although generally I agree with the engineering references, I disagree with the claim of no information from torque curve readings.
    Horsepower numbers (to most street drivers – I’m not talking racers here) are just flashy numbers to impress people. Horsepower numbers are the cool tattoos you have to show off when you flex your bicep. Torque numbers ARE the bicep.

    Beg to differ. Our experience shows that a cam change results in a particular horsepower curve, but when a power-adder is installed, both the horsepower and torque curves change noticeably. In the case of a supercharger installation, the torque numbers change dramatically while the hp increases but not as much as the torque. In any case, these numbers are relative at best, because the vehicle dynamics aren’t in play. Wind resistance (drag), rolling resistance, vehicle weight and gearing and road conditions affect the power output to point where “X” horsepower and torque numbers change enough to make those a moot point. Does the vehicle perform better? Does the fuel economy change? These are what matters. And our shop would discourage dyno time on our engines because our customers couldn’t drive a dyno, but they could drive their cars. The dyno just added wear time to the engines. If an engine is machined correctly and the proper parts are installed, the results speak for themselves. If we built an engine capable of making a respectable power number, that number is only felt at peak operating RPM as you said. We built street engines for torque and durability, and came with a 50,000 mile warranty. Our racing engines were built for maximum output for the duration of the intended usage. And they carried no warranty. We would tell people that they would turn 10,000 RPM. Once.

    how a lot of horsepowe afficinados arrive at the H P of their Engine (maybe?0 ) Take the Factory avertised HP and add x amount of HP for more expensice Spark Plugs then ad the gain in HP by more Expensive Airfilter then ad the added Hp by a Custom Exhaust System after that add the width of te front Bumper squared by the No of Cylinders in your engine (MAYBE?)

    Duh! Shows how poor my math skills are that I had to look at the DYNOJET graph for my Stage 1 Golf R to prove the point. I just thought that convergence was specific to my car. Duh.

    One thing to note about octane ratings on gasoline. Even though your engine may be made to run on 87 octane, you may want to get 92 for a car that sits a lot IF the gas doesn’t have ethanol in it. It doesn’t help the running or HP, but it can prevent the evils that occur from gasohol eating the rubber hoses and corrosion of parts. Alcohol is for drinking or cleaning parts. It has NO business being in gasoline. It’s just a ploy by the farm lobby.

    Joe, I always thought ethanol free was the best fuel to use when storing a vehicle. What am I missing or misinterperating?

    That is correct. Most gasoline in the lower octane ratings have ethanol in it. I have ONE station near me that sells premium 92 octane that does not contain ethanol. It costs over $1 more per gallon than regular grades that contain ethanol.

    My station sells 89 octane ethanol free at about .50 cents more a gallon, well worth the cost vs any expense to replace hoses etc. I use it for every fill up riding or storage.

    Exactly Joe! Thank you for saying this! I was going to comment, because how can you discuss octane and not discuss the poison that is now added to pump gas for political purposes. Only modern, all stainless steel fuel injected systems are appropriate for this toxic liquid. In addition to listed problems it will destroy carb castings in time and has a lower vapor pressure, leading to potential carb fires on hot days, and has a very short shelf life before it turns to Jello.

    Never be tempted to put gasoline with any amount of alcohol in any marine engine. Just don’t. Alcohols are rabidly hydrophilic, corrosive, and as solvents have no inherent lubricating properties. Better to just burn the boat to the waterline than wreck the engine with alcohol.

    “For Sale: 1974 Z28. Make offer – must sell. Apparently ‘Do what you want’ doesn’t mean what I thought it meant.”

    The cold hard truth is that everyone has 500 HP. I deal with customers and often ask what their HP is and they all say 500 HP LOL!

    It is always fun to watch them get on a dyno and find the real power rating.

    You know the basics are this. Know what you will take for your car to make it worth your while if someone comes up and wants to buy it.

    As for gas you have a gauge use it. As for Oil pressure know what is normal as too many see the gauge and start to panic as they have no clue what normal warm is at what RPM. That is why most cars haver idiot lights.

    If you have a modified engine know what your compression is and what octane is required. Don’t pay for more unless you need it. And if you need it make sure you have it. Also know where the timing needs to be.

    Know what the speed limit is when the cop makes a U turn behind you. Yes they will make a U turn and follow you if you have two carbs out he hood even if you are 10 MPH under.

    If you have an engine from another vehicle in your car. Know the year and what it came from. Do not assume it is a 4 bolt main Corvette engine as most are not.

    I could go on about numbers that really matter.

    #1 number to have. If you are calling about an order to Jegs, Summit or Speedway have the invoice and number with you. Don’t call with out the number or a clue what phone number the order is under.

    Good point about knowing what engine is in the car if you did a swap. My mom had an ’83 Bonneville wagon (RWD) that originally had a Buick V6 (blew up on vacation>>long story). I put a ’73 Chevy 307 in it and it had many different years of accessories and pulleys on the front so I had to measure belts for it. Not to mention the removal of the computer carb and putting a 2bbl from a ’79 305 on it. Tried Quadrajet, but it was bad.

    A chassis dynamometer measures rear wheel horsepower which is less than engine horsepower because of friction losses (heat) in the transmission, rear end, tires, etc. However, rear wheel horsepower is the number that moves the car.

    In the ’90s, the marine industry began to publish prop horsepower instead of crank horsepower. The published horsepower of the Mercruiser (Chevy) 427 outdrive dropped from 330 hp to 290 hp.

    I always love the dudes with no car at an event that go around nit-picking other guys rides (they usually showed up in a Camry or something similar) and generally acting like a know it all. I always made a point of asking them “Where’s your car, bud?” and couldn’t wait for the discombobulated answer. I know that’s not real nice but it was gratifying!

    I did the Concours judging for 30 years for a old car club in South Africa and I can’t tell you how many club members, and visitors, would come to me and tell me what was wrong with the cars being judged yet not ONE of them that I can remember EVER restored a car let alone put one on Concours for judging. I had a big argument with one guy who said that a British 1961 Mk 2 Ford Zodiac was actually a 1959 because the tail light lenses were manufactured in 1959! Dum bass didn’t even know that all Mk 2’s from 1959 until 1961 had the same lenses. Another guy told me that my 1912 Little was some other kind of car, the make of which he couldn’t tell me, and said that I must have had the enamel “LITTLE” badge manufactured to suit the “little” car!

    Such folks exist. I have a 1940 Cadillac Coupe in fine condition and I had some guy stare at it for a long time not interacting with me whatsoever, and then tell me that the central crest medallion at the top front of the grill wasn’t exactly centered, make a big deal of it with everyone in the vicinity, forcing them all to see what he’d spotted, then walk away.

    The gauges in my Tundra PU, AH 3000, and FacFive Cobra are all REAL. Yes, they fluctuate but one “learns” where they belong at any given parameter, and can save an engine. The modern equivalent of a “fake gauge” is the VOLTMETER. AMMETERs measure current flow, so they can INSTANTLY tell you if you’re charging or DIScharging, but a Vmeter won’t show a significant drop for a while…. awww, but they’re cheaper to install ya’know.

    I disagree with Thomas about the voltmeter. Yes it is cheaper to install but it is almost as quick to show that the alternator is not charging. It will read about 14 volts when charging and about 12.5 or less when not. Also I do not relish the thought of all my cars electrical needs and current running through one unprotected wire under my dash. The voltmeter is just a much safer option

    Voltmeters will show trending – if you check it once in a while while scanning the dash, that should save your bacon. If it fails, nothing bad should happen. My AH MkII had an Ammeter that shorted while my now-wife was driving it. Fortunately she had the sense to stop the car and open the battery switch before it flamed.

    Yeah, I had the ammeter go bad on a T bucket. There was no real indication it was bad but the next thing I knew the engine just shut off. Battery was dead. I don’t believe the ammeter was wired correctly as I had to rewire the electrical in the bucket. I replaced it with a voltmeter, wired correctly, and all is well.

    I agree with Dan. In 1963, I worked in the GMTC (General Motors Truck & Coach) electrical lab. We always set the voltage regulators at 13.7 v.

    I disagree. Start your voltmeter -equipped car. Turn the headlights on, note the reading. Turn the engine off, leaving the lights on and note the voltmeter reading. You will instantly see remaining battery voltage. An ammeter will show a discharge (negative reading) but won’t tell you how many volts are left in the battery. Both types of gauges provide information, but the voltmeter tells you how long before you start walking. A fuel pressure gauge tells you that you have fuel pressure. A fuel level gauge tells you how much fuel you have. Kinda the same thing.

    They also burn your car to the ground, sometimes. Ask Mopar owners.

    The ideal solution is what GM installed in the late 60s; microvolt meters. They measure the voltage differential between your battery cable and alternator, and can thus show charge/discharge/equilibrium.

    Low current on a shunt versus directing *all* electricity through a small gauge.

    I thank the engineers that were figuring this stuff out decades before I was born… 😉

    I’ve had real gauges in all of my classic British cars. My current MGA 1600 runs 60 PSIG when cold and 40 PSIG when warm. My Harley Roadglide, while it doesn’t have a gauge, has a has a screen in the information display and when it is cold it will run 50 PSIG. When hot at speed it runs around 36 and idling it is closer to 20. I for one understand how temperature and RPM affect oil pressure and these fluctuations have never bothered me. On the BMW idiot gauge, that only confirms my thoughts about most BMW drivers….;-)

    Bob B. My ’78 Spitfire used 20-45 oil. One time I put in some 10-30…yikes after a minute as it warmed, almost no oil pressure at idle. I guess a lot has to do with bearing design. I think the Spit 4 banger was once a tractor engine. Talk about horse power…….. all 55-or 60 of them

    That’s a good one, Drew! I’d also add knowing how much oil (and what grade/viscosity) your engine needs is important for both oil changes as well as top offs (which could be needed while away from home so you should know what oil to buy). I’d say that and tire pressure are a bit more important than knowing the arbitrary value of your vehicle…

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