Most filling station gas pumps underregister the gallons to avoid code
> enforcement fines. Therefore calibrate the gas pump by pumping gas into a
> known calibrated container. This error will falsely increase your > calculated mileage.
>
> Most odometers are wrong. Calibrate that against road markers. An
> overregistering odometer like mine will also false increase calculated > mileage.
>
Odometer inaccuracy is a serious problem and very common, far more than people realize. The automakers have a vested interest in making odometers read a few percent high with all the leasing that is done and the mileage warranties. See the following rather interesting discussion:
formatting link
Road surface conditions, water or snow on roadway, load in car, winds,
> grades are variables which affect mileage.
>
> Less critical is temperature and atmospheric pressure, and altitude. >
> Tire pressure is often overlooked.
>
Tire size is more important.
The professional mileage contestants pump up their tires to about 60 psi
> for optimum mileage. They don't care if the tires blow out. Its the
> mileage that counts, dude.
>
> Some of you guys out there think you're getting 35 mpg from your Imperial,
> but you aren't because all of the above confuse the issue.
>
You forgot winterized fuel with a higher alcohol content, thus lower power from it, mileage on these blends is worse.
It should be noted, however, that MPG rates are most useful in comparing PAST mileage with CURRENT mileage, as a decrease in mpg indicates engine trouble. SO most of these factors are equalized out. mpg only becomes significant when comparing it to other vehicles or against the federal tested mpg.
Ted