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Am I the only one whose computer mileage readout is all over the friggin map compared to actual mileage? I just filled up this afternoon, and despite the computer saying 50.3 for this tank, I actually calculated 42.7 (a *HUGE* difference to my way of thinking). The tank *immediately* prior, the computer showed 48.2, and I calculated 50.1! What gives? I know there is usually a discrepancy (mine has averaged 8%), but the computer almost *always* reads higher than the actual calculated. How can it be so variable when I'm doing the *exact* same driving in the *exact* same conditions? Nothing changed from one tank to the next. Same gas station, same pump, same gas (presumably), same routes to and from school & clinicals, same everything. Wildly different results. The scientist in me is perplexed.
Mine is always off a little, but the computer is always low. Sometimes by as little as .4 MPG, but the most off was 2.5 MPG...again, calculated has been higher by usually 1MPG on every tank. I'd prefer it be low than high. Never had it be very far off, though.
By actual mileage, I assume you mean mileage calculated by topping off the tank manually? Any single measurement by that method may not be particularly accurate either. Over time, it probably becomes a more accurate measure. The computer measurement may be off, but it's probably off in a more consistent way, making it useful for determining mileage change as per drving behavior change.
IMO the computer is best for comparing one tank to another. The difference you speak of is most likely due to fill inconsistancies. These errors average out over time so the "at the pump" method is best for computing a long term average.
My display is usually around 3.5% too high so I use fill data when reporting mpg to the database.
The difference you speak of is most likely due to fill inconsistancies.
Lakedude is right on. Each fill-up is different, so as mentioned earlier in the thread, unless you fill up manually where to the top of the filler tube (not recommended), you will get discrepancies from tank to tank.
How much discrepancy obviously is a factor of how consistent one is with filling to the exact same spot in the tank.
I don't know how consistent the computer's mileage is, but I bet it's no worse than the inconsistencies introduced by different tank fill levels, so maybe we should not be complaining about inconsistencies in the computer, but inconsistencies in the fill level from tank to tank.
Case in point, usually when I fill up it's at the same station, even the same pump. I fill it on "medium fill speed", then two more clicks at very low speed and I'm done. My mileage difference between computer and computed is usually about 2 mpg. This last fill I went to a different station and did the same routine. But the second click at low speed didn't click off like I expected. It kept filling, and I let it. By the time it quit, it had added another gallon and a half. The result? The differnce between calculated and computed was 8 mpg! Following a fill, I usually get between 50-60 miles before I lose the first bar on my fuel display. This time I'm at over 80 miles since the fill up, and the display is still showing all bars. I don't blame the computer for this, I blame the fact that I filled the tank higher than I usually do.....
I finally filled my tank for the first time over the weekend after having my car for a week. I was able to get 518 miles (still had 2 bars left) but the computer was showing I had 41.3mpg but when I calculated by hand I got 50.1 mpg. Are my calculations more accurate?
I did notice a huge difference in how soon I lost the first bar of gas. When I picked it up from the dealer I got close to a 100 miles (or I may have even reached 100) before losing the first bar. However when I filled it up (put in 10.3 gallons) I noticed that I lost the first bar much much quicker, so I know that I will not be able to get as many miles on this tank probably.
I want to start logging my milage on this site but haven't yet, should I use my calculations or the computer?
If you had 518 miles with two bars left, then yes, your calculations are far more accurate. There will be small variances in the accuracy of the calculations at the pump (per tank), but over several tanks, it will be FAR more accurate than the computer, because it's your actual fuel cost into your actual driving. The only variance is actually the FIRST tank fill when it was filled before being driven. Over the course of the car's life, any 1 gallon discrepancy from the first fill becomes so minute that it will not affect the average at all. I have noticed that the computer is .5 to 2.5 MPG low...never had it more than that.