Correct for incorrect miles due to tire being some percent different than the trip computer was calibrated for. Maybe possible differences due to short trips & many cool downs for a given tank of gas. Possibly, the use of 100% ethanol-free gasoline instead of 10% ethanol blends may cause differences.

Not quite the same, but my mpguino under reports my mileage by a bit. I decided not to recalibrate it, so I'm just nicely surprised each time. It's off by just the right amount, actually: if I put in a bit of effort, I can get a trip (and sometimes tank) average of over 50mpg according to the computer. This translates to about 53mpg at fill-up.

I'm finding the trip computer to be pretty optimistic in my new Seat Leon. I'm keeping a close eye on the discrepancy as it seems to be getting worse with time and the car has only done 2000 miles! Last fill the difference was 4.5mpg :confused:

Well, I think we're coming back down to earth. With the latest fuel-up logged, my average is 17.1, with the trip computer reading 17.4.

I'd love to find a station with 100% gasoline, if only for my lawn equipment. There are some around here, but they're all out in the sticks.

I suppose there must be a number of different "trip computer" systems out there, but you need to make sure that you're operating yours correctly. For instance, my system's "Cruising Range" number is based upon the "Average MPG" number, and you really need to comprehend that you need to RESET the "Average MPG" setting each time you fill up, so it can calculate both the Average MPG and the Cruising Range for each tank of gas -- otherwise, both numbers are going to be relatively useless.

So, make sure you understand how to operate your system, to begin with, and also make sure you comprehend WHY you should operate it one way or another, to get the results you want from it.

I don't believe my trip computer operates that way. It's a pretty simple one compared to some out there.

Yes, that's what I was trying to say -- there must be a ton of different systems out there, and they are most likely very different from one another.

Yes, my trip computer is quite simple compared to the ones in newer vehicles. The trip odometers are the only things I can reset. What I really wish it had was an "instant MPG" readout. You can use those as sort of a "driving coach" to optimize fuel efficiency in all situations.

I have found in practice that instant MPG displays are next to useless. The only time they give you any useful data is on flat ground at perfectly constant speed (i.e. with cruise control on). Any wind, slight incline, or the slightest acceleration or deceleration will give you bad data. There's no way to judge how efficiently you accelerate, coast, no way to get any data at all unless you are on flat level ground at constant speed.

I had hoped that the scangauge was smart enough of a tool to have figured this out... but it was mostly useless. It has instantaneous MPG and trip MPG and no other options. The rolling average is what's really useful, and a combination of trip MPG and tank MPG is very useful as well. I sold my scangauge after a year or so... waste of money.