What level of accuracy are you using for yout testing?
I know a lot of people simply don't go to any trouble, but they work out their figures as if they're facts.
You need to check your odometer over a decent distance, 10 miles is good. You need to fill the tank right to the top and record the mileage to the tenth.
And then, if you have mile pegs on the highway, sit on a steady 60mph by the speedometer and time the mile with a stopwatch. Divide the number of seconds into 3600 and that will give you your exact speed.
