Quote:
Originally Posted by Dadhawk
Both methods are subject to some level of error. Using the fill and calculate will tend to run a little low over time because of the differences in what 'full' means based on how the car is sitting when filled and other variables (you are more likely to underfill it to "full" than overfill it). Depending on whether or not the computer uses real fuel flow versus some other estimate it can have its own share of errors.
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Actually, they aren't subject to the same error. The car calculates fuel consumption based on, essentially, a whole bunch of sensor readings, and determines what it *thinks* you are getting. This is an "ideal world" scenario where engineers have determined a formula for the vehicle to use. The car can't account for all parameters, so the formula is an estimate. It never actually KNOWS exactly how much fuel it is using, nor does it actually ever know how much gas you put in the tank. It can only estimate those values.
Calculating fuel usage yourself, however, is extremely accurate, as you are eliminating a lot of potential for error, and are left with a few basic assumptions:
1. Is your odometer accurate? For all intents and purposes, yes. Assuming you aren't using the trip meter, it's only accurate to the nearest mile, but even across one 30 MPG tank, that will only throw off your estimate by a small amount.
2. Is the pump dispenser accurate? For all intents and purposes, yes. LEGALLY, they are required to be INCREDIBLY accurate.
As far as whether or not your tank always fills to the same level, you are correct, there are differences based on how your car is sitting, and how sensitive the pump shutoff is, however these errors only matter on the very LAST fill-up that you are including in your measurements. If you are calculating an average across a bunch of fuel-ups, then the errors introduced in-between don't mean anything. You know how MUCH fuel was put into your car across all the stops, you just can't be sure that your last fuel-up accurately states exactly how much fuel you used since the previous one, because it might not have filled to exactly the same spot.
Essentially, all of these potential errors, if you distribute them over several fuel ups, become statistically insignificant. If you do the math over my 25,000 miles, or your 77,000 miles, you can be pretty damned sure that the number you come up with is pretty much EXACTLY what your actual average fuel economy is.
This is, of course, assuming you can properly read the pump, and record the numbers, and do the math without screwing up, lol
