Adding extra weight to your car will slow acceleration, but won’t reduce its top speed. In determining a car’s top speed, its engine battles two main forces: rolling resistance and aerodynamic drag.
At very high speeds, air resistance makes up the vast majority of the overall drag on your car. So much so that an increase in rolling resistance from extra weight is likely negligible.
Adding weight may increase your car’s rolling resistance, but it’s also usually offset by a decrease in wind resistance because the weight compresses the suspension, which in turn decreases the car’s frontal area and, typically, its drag coefficient too.
So in some cases, your car may be even faster when fully loaded — at least on level ground. The extra weight will increase top speed going downhill, but decrease it going uphill.
But be careful of things that seem like they’d be related. Like, don’t believe someone when they say the only sure way to make it rain is to wash your car. Because no matter how much it seems to be true, it’s not.
Oh, but your car might require less stopping distance when fully loaded. See? Sometimes things are related in the most unexpected of ways.