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’re checking octane recommendations, best keep in mind that the methods for measuring octane have changed over the years. Don’t blindly depend on the original books for a car made in the ’60s, for example. The same thing is true even more so for horsepower. In addition to the changes made in measuring methods, manufacturers blatantly lied about output for several reasons, including keeping insurance costs down.

    Horsepower is torque at work and a derived number from torque readings at a given RPM. If you have either you can compute the other. Horsepower=Torque*RPM/5252. All a dyno measures is applied power at a given RPM and that applied power is torque which is later converted to horsepower numbers through the above formula. You can construct a horsepower curve having the torque curve or vice versa. The math formula guarantees that the constructed torque and horsepower curves will always cross at 5252 on the dyno printout as long as the engine can rev that high.

    Octane requirement is one of the most misunderstood things to do with gasoline engines. Some (not all) manufacturers have added knock sensors to detect when pre-ignition occurs, and to adjust ignition timing to mitigate it.

    These engines require high octane fuels for optimal operation where the extended combustion cycle burns fuel more completely, yielding cleaner exhaust and more power. The longer combustion cycle results in less stress on many of the moving parts, especially bearings and seals, for longer life.

    The thing about this that confuses so many people is that there is not much consistency in octane requirements, even within the same manufacturer or division. Many people take offence to being told by the manufacturer they need high octane fuel, because there is so much misinformation about this circulating in social media. Bottom line is that the manufacturer is the only authoritative voice on the subject.

    Three of my cars are old enough that they required leaded gas—not available anymore and not mentioned in this article. None originally called for a higher-than-“regular-gas” either: Just the lead. For a very short time after the lead disappearance, Shell made a big thing about being kind to its customers by adding some secret lead-substitute formulation. But not long after, they stopped that, and claimed their new gasses were the best for most people (the percentage of lead-requring cars supposedly gradually declining, so all of a sudden, to heck with those old-car owners). Botttles of lead substitute additive are widely avaible. However I had a long-ditance phone call with a technical official at a majotr refinery; I was told that “lead substitute” is not usefu. Period.

    I use Shell V-Power in the 3 old cars that sit around alot, because here in canada, it never has any rubber-rotting alcohol in it. The 3 do not call for premium, and yet my engines have not been noticeably self-destructing, as warned in the article. Also, a Volvo dealer/friend told me that octane drops over 5 months or so sitting in the tank—not that that matters, apparently, with these cars. So they get the V-Power alwars. My son’s ’99 Volvo V-70 recommends “Premium” gas. But does not require it—another hitch to the story of gas (“recommend” vs. “required”).

    My five cars, which includes two comtemporary ones, always get Top Tier gas, which in southern Ontarion means all grades of Esso, Mobil, or Shell. My Chrysler handbook nags that Top Tier should be the gas of choice—and so does Consumer Reports. This Hagerty article, prodding readers to baby their engines with the correct gas, didn’t dig into these aspects.

    Driving for a weekend will not give you an accurate fix on fuel consumption. Small variables such as gas pump shut-off sensitivity, and type of driving (number of stops, idling periods, etc.) will skew your results. Drive for what you feel is a tank’s worth, minus a safety factor to get a better result. The same errors as mentioned will be there but will not affect the calculation as much.

    The shunted amp. meters in GM vehicles (usually trucks and pick ups) in the late 60’s and early 70’s were all too often so dead as to show no movement at all. Maybe running all the current through one wire to the gauge has it’s drawbacks but that type of gauge was more reliable by far. That wire running to it was protected by a fusible link and not unprotected as someone stated earlier. As far as oil pressure readings go I usually add a gauge to my vehicles. That way a light will get my attention and a guage will verify the results plus showing the lowering pressure due to wear of an engine as the years go by.

    About the oil pressure gauge vs idiot light. I firmly believe they both have their place. I my vintage cars I have the factory light functioning plus I have added a gauge as well. The gauge gives me the ability to monitor any reduction in normal pressure readings; but you can’t be monitoring pressure at all times while driving. If there is a catastrophic drop in pressure while driving I may not happen to be looking at the gauge but the light will immediately illuminate and at least give me a chance to avoid a complete engine failure. The added benefit is that on start up after not driving the car for an extended period of time I crank the engine over until the light goes out before pumping the throttle to fire the engine that way I know I have lubricated the bearings properly before ignition.

    Keith, good idea about cranking the engine before touching the throttle pedal to set choke and squirt fuel to start the engine. I am going to do that next time I fire mine up while setting during the off season. I may put a note on the steering wheel to remind me.

    Totally agree with most of you: torque is measured on a dyno; horsepower is calculated. And an 8th important ‘number’ is how much RESERVE fuel is in your tank, if any. Having the reserve select lever in the wrong position bit me on both a Porsche Speedster and a Yamaha 400, both resulted in long walks.

    Lively discourse, as this platform was designed to encourage…

    The Horsepower/Torque debate will rage forever; Acceleration and Top Speed are the results; go figure which produces what…

    In a quasi ‘Spec Car’ Series many years ago, we arrived at an option package for a Roadrace Series that would produce ~500hp in retired ASA Cars. Most were 495-515, but mine was 472; however, with my ‘fatter’ Power Band (don’t hear that term much anymore), I could come off corners quicker, use less ‘gear’, short-shift transitional sections, use less brake and less fuel (handy when we went Endurance Racing) than anyone else; worth considering in a Street Car…

    The same car was only equipped with mechanical Oil Pressure and Water Temp Gauges (not even a Tach, as gearing was already calculated and optimized for each Track), presumably to let the Crew Chief know what had failed first, in case of mishap…

    What we added, was something we had ‘pioneered’ in the mid-70’s. Mechanical Gauges from quality manufacturers (SW, VDO, etc) were very accurate, in real time, but didn’t tell you anything, until you looked at them. ‘Idiot Lights’ only informed you after the fact. By using a ‘T’ fitting, and researching the thresholds of various on/off sensor offerings; we were able to get the accuracy of the gauge, with the Early Warning Signal of the Light next to it. We also rotated all gauges to point at 12 o’clock when at nominal readings…

    Mostly ‘cuz we looked at what Old Guys were doing, and why…

    The Parts Guy in me also wants to remind ALL my Motoring Brothers and Sisters to please, please know what year, make, model, displacement, and any other pertinent and accurate info when ordering parts for the object of your affection. Yes, we ask a lot of questions, because we want to provide you with a successful result, and, ‘No’, they are not, ‘all the same’…

    If you have a pre 1966 car the VIN can be a problem when dealing with the DMV… in many cases pre 1980. The current 17 character VIN became mandatory (for US cars… all this info is for cars sold in the US) in 1980. Prior to that there was a 13 character VIN that became mandatory as of January 1966 that had a standard format for all manufacturers. I’ve tried to register a late 70s car wand been told “there aren’t enough characters” in the VIN. I explained that 66-79 only had 13 characters, and a supervisor had to be called to verify. AMC/Rambler used only a 6-7 character serial number before 1966. Take that to the DMV and see what reaction you get! Prior to 1966 manufacturers used whatever numbering system they wanted. A few had started incorporating more info than just a unique serial number (the only requirement prior to 1966). I’m an AMC guy and most familiar with AMC numbers. Starting in 1958 AMC had a letter identifying the model followed by 4-6 numbers (by 1960 always six numbers). In 64 the single letter represented the engine type/size as well as model. Early production 1966 cars have a serial number only (model year production usually started in late August or early September of the previous year). A handful of early January models have both a serial number and the then new VIN — they had been produced with a serial number but not ready to sell until January, so had to have a VIN.

    AMC did something different with their VINs than other US manufacturers (AFAIK no others did this). The last six numbers are always a unique number for the car on all makes. Most US makes use that as the production sequence. AMC assigned a VIN as the cars were ordered, not made. So VIN 000001 might be the 10th car off the production line. They used the VIN to track orders, not physical cars. On the bottom of the door tag that records body, model, trim and paint numbers is an unlabeled number. That is the “Final Assembly Number” — the order the car came off the assembly line, not the VIN number.

    Aww, man – you made it all the way through that until the last sentence without saying “VIN number”! Almost a terrific job, Frank, but sadly, we must deduct all of your winnings. Thanks for playing our game, though! 😆

    If you have a Corvair you’d better know your tire pressure and it’s not always the number in the owner’s manual since that’s for bias-ply tires.

    At car shows, I often ask owners, “How much does the car weigh?” Most do not know. Power to weight ratio can give an owner some idea of the car’s capabilities. Trans type and gears are also important factors. My car has 330 hp, weighs 3100 lbs. and proper gearing that outperforms many more expensive and so-called fast cars. Colin Chapman made a name for his car designs by watching his cars’ weight. Why do you think carbon fiber elements are so important today?

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