What Car Q&A May 2009 - How do I measure my car's mpg?
First, fill your car to the brim. The more slowly you do this the less chance there is of airlocks developing in the filler neck or fuel tank. Next, reset the trip computer so the mileage count reads zero. You don't need to do anything else until the tank is nearing empty.
When you next fill up, do exactly the same thing, but make sure you jot down the number of litres required to refill the tank (this should be displayed on the pump or on your receipt). Also record the number of miles covered before resetting the trip computer again.
To calculate the MPG, you need to convert the number of litres used to gallons (just divide the litre-based figure by 4.546). Then divide the number of miles covered by the number of gallons used.