Here's a little math lesson on how to calculate mileage rate.
1. Take note of how many miles you have driven on this 1 tank of gas. for example, 100 miles.
2. Fill up the tank completely and make a note of how many gallons or liters (for those metric ppl up north). 10 gallons
3. Divide the mileage by the gallons/liters and bamn = mileage per gallon/liter. 100miles/10gallons = 10 miles/gallon
It's that simple. Don't question how that come about, b/c I don't think you can handle the "real" math proof.
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For the ppl that can actually do algebra.
Distance/Volume of the tank = mile/gallon or km/liters
Now about the issue of what # to use for the volume of the tank. Use your common sense, if you use completely the 14.5 gallons that is available to our MSP, then your car would die precisely at the pump without you turning off the ignition. Why? Because the engine has used up all the gas. Now, I presumed you're a good driver and don't do that, cuz as a MSP owner u know at least basic rules of taking care of a car (crossing my fingers). Therefore, you don't use the complete 14.5 gallons of fuel, you use less than that. The fuel warning light comes on before you run out of fuel, that's why it's a warning light to let u know hey driver I'm running low and I'm gonna die on you if you don't fill me up.
Another way to prove that you don't divide the whole tank is fill your car up and look at the volume of fuel you pump in. This is the amount of fuel you use since the last time you fill up. Now if it's 1 gallon, would you still divide the mileage you drove with the whole whooping 14.5 gallons to get a dismal say 1.5 miles/gallon(22/14.5)? No, so this proves your logic wrong. I hope you understand this.