Calculating Gas Mileage?

[rewecow]

I am looking for a formula to calculate a car's instantaneous gas mileage with variables such as friction, weight, air resistance... if someone knows it by heart or could lead me in the right direction to find it, it would be much appreciated.

[Rexxrally]

Impossible, dude, there's too many variables and too many unknowns.

The only way to do it is by the tankful and the miles put on since the last fillup.

[rewecow]

It doesn't need to be exact, I need more of an approximate shape of its gas mileage vs speed or rpms (by the way its the Lexus RX 300)

[rewecow]

I'm looking for a chart of instantaneous gas mileage vs speed or RPMs... I'm thinking now if I can find that chart for any car it would be nice.

[Dr. Lightspeed]

Even more factors to consider. Pulsewidth for your injectors is going to vary based on load. You could calculate how much fuel you were using by measuring the pulsewidth and doing some math but that is not going to be something you can calculate by RPM as pulsewidth is going to varied. The best way is to fill up at a gas station on the outskirts of the city. Put the pump on the last click let it click off by itself and do not try and fill it anymore. Then go for an hour drive one way and then back. Go to the exact same pump put it on the last click and once again let it shut off automatically. Record your mileage then based on volume of fuel and miles travelled.