Disclaimer: I am not liable for injury, death, or damage from the following. Please suggest safer, better ways of measurement. Measuring MPG. Fill up. Drive Y miles where Y > 200. Fill up, and record number of gallons filled X. Divide Y by X. Ideally do all this on a single road trip to avoid temperature effects. Measuring tank capacity. Keep a full 1-gallon gas can in the car. Drive until red triangle indicates you've run out of fuel. Drive another quarter-mile until you're on fumes (this damages the car, yes). Pull over and fill up with your 1-gallon gas can. Go to gas station and fill up, and record number of gallons X. Tank capacity is X + 1.
You need to add at least 3 gallons for the car to register that fuel has been added, so you need a spare gas can of 3 gals or more.
