# Calculating gas mileage

What’s the best way to do this? I prefer to note the odometer reading with each fillup, and divide the mileage by the gallons I get; but sometimes I can’t afford a fillup, or don’t have time, so I get only a few dollars’ worth. Some mechanics (including a station owner I have known for 30 years) say it’s not really possible to reckon gas mileage accurately when a small amount is added between fillups, even if the total gas added, the small amounts and the fillup, are toted up. Noting the mileage at each time I even add a small amount, perhaps?

As long as you have some record of miles (or kms) driven and gallons (or liters) of gasoline (or petrol) used, the quotient will give you mileage per gallon (or km per liter.) Doesn’t matter how many times you fill tank or just add partial amounts (as long as you keep track of what the partial amounts are.)

You don’t need to keep track of the mileage at each fillup, either.

Take odometer reading at end of test, less odometer reading at beginning of test; that’s the number of miles. The intermediate stops are irrelevant.

Start with full tank of gasoline at beginning of test. Keep track of total amount of gasoline you put into the car throughout the test period, including the amount required to fill up the tank at the end of the test. The total number of gallons of gas is the denominator of the fraction.

The reading will be off a little, of course, because you won’t have an exact reading on the amount of gasoline you put in the car. And “full up” is not always the same each time. But, close enough for practical purposes.

Thanks, Dext.
Now I’d like to know, if you or anyone else can tell me, what the gas tank capacity REALLY is on an '84 Thunderbird 2-door sedan, which is what I drive. The owner’s manual has addenda in it and I’m not sure if the 20.2 gallon capacity printed in the Ford manual is accurate.

When I do this I usually use the exact same pump for the first fill-up as I do for the final fill-up. And I don’t top off at all, I just let the pump stop itself. I’ve always figured that this was the most accurate way to do it. Any other techniques?

This brings a potential SD question to mind: How does the pump know when your tank is full? Somehow I doubt that it stops when the fuel level has gone up the filler spout and come into contact with the actual nozzle of the fuel-filler. If it went up that high, wouldn’t it be in danger of sloshing out?

I’m not sure about that last question, but I do know that you have to keep the tip of the nozzle away from the level of fuel when you fill a gas can! This can be maddening–the way the pump keeps clicking off–unless you learn just how to do it, be it can or car.

I believe that gasoline nozzles contain a valve that senses back pressure. Once your tank is nearly full, enough air starts getting forced back that triggers the valve.