Its seems from my records that the mpg gauge is more accurate on short trips than long. For instance I went 150 miles, filled up and the gauge was less than one mpg off. (See my post in computer vs calculated) Than on a long trip the gauge is off usually 2.5 to over 3 mpg's. As anyone else notice this? alfon
Why would this be a logarithmic scale? Error should accumulate in a linear fashion: twice the driving, twice the error, same percentage. Tom
Changes in temperature, weather, driving distance will always modify what the actual reading will turn out to be. Since I started tracking fuel consumption a few months back, I noticed one particular fuel-up which was 286.4 miles and the calculated & actual MPG were exactly the same. Came around to 59.2 MPG. So I wouldn't necessarily say the estimate will 'always be a little off' given the distance traveled. Just be happy you are behind the wheel of a great automobile taking you where you need to go!
I have a similar question. I am on my 3rd tank. The first two tanks the trip computer registered at 580 miles. On the 3rd tank, the computer is only registering 495 miles (and I filled it to the rim). Why such a difference?
Actually, I think that the MPG error (hand calculated vs. computer) should be more stable (not higher or lower, just more stable) for long fillups rather than short fillups. Here's why I think so: The idea is we try to fill our tank roughly to the same point each time. The pump will click off at roughly the same place +/-. The error +/- on any given tank we can call the single tank fill error. It will have an average, variance, standard deviation, etc. There is no reason to believe that the average single tank fill error will be different depending on whether we are adding two gallons or nine gallons (tell me if I'm wrong here--those of you who know how tank filling works better than I do). So, the average single tank fill error should be a smaller percentage of a full tank fillup (say 9 gallons) than of a partial fillup. Thus, the hand calculation of mileage should on average be closer to accurate on a full tank fillup than a partial fill up; and the difference between the computer and hand calculation should be more stable with full fillups.
Something I'll do that will give a damping effect is to stop and pump a gallon. I usually do this while picking up a newspaper. Of course I'll add this gallon in when recording mileage on fuelly.com. As a rule, I ignore the mileage to fuel up numbers on the display. I can do the math and know how close to approximate I can go.
