Measure the distance (off the odometer) and measure how much fuel you put in at the fuel station.
Rather inconveniently, we buy fuel in Ls, but use it in Gs, 1G=4.546L, so if you want your mpg, go fill your vehicle up, and take a note of the odometer reading. Drive, fill it up again, then work out the distance. The fuel youve used is the amount youve just put in. Divide by 4.546 to get it in gallons. Then divide the miles driven by gallons used for MPG
Who are you trying to teach how to suck eggs...where is your formula for a knackered series engine with a worn carb badly adjusted and a whiff of fuel from corroded fuel pipes...