Built-in MPG calculators - Is yours as much of a liar as mine?

[nitrogen]

I thought I was doing well, going by the built in computer in my car.

nope. Apparently it's a big ole liar. What's up with this? Are other cars' built in computers liars too? Is the scanguage better about this than the built in computers?



EDIT: It's a liar, telling me I'm doing BETTER than I really am, to clarify...

[forgottenmindset]

How much was it off by?

[nitrogen]

Originally Posted by forgottenmindset

How much was it off by?

5.5mpg.

[Who]

I've never heard of one ever being pessimistic, but that's a huge diff!

[instarx]

My Volvo's computer consistently indicates worse average mileage per tank than I've really gotten. it's usually off by about -2 mpg. The speedometer and odometer readings are exactly right as measured by my GPS, so it isn't some strange tire diameter issue.

I would guess that OEM computers use a standard factory calibration value that can be a bit +/- for any particular car (but 5.5 mpg does seem like a lot). An after-market guage could be better or worse, depending on how well you calibrate it.

[nitrogen]

See, mine tells me I'm doing BETTER than I am, which is even MORE confusing.

I wonder, since I run Regular, and it likes Premium, if it doesn't take the automatic timing retardation into account.

I'll be interested to see how much better I can do with a scanguage.

[extragoode]

Our Jeep is the same way. The average reading will say 13-14 and miles traveled divided by gallons is more like 11-12. Sometimes it's better than others. I started reading up on why and found out that the average is only the average over the last 50 miles! So if I drive it for the first 200 miles then my wife, the last 50, it's about on, since the readout is still optimistic. The other way around, it's just WAY optimistic.

[azraelswrd]

I remember reading some posts on CleanMPG that Toyota built-in MPG meters are notorious for being too optimistic (ie higher MPG than it should be). Not sure what would account for these discrepancies but this is why nothing beats the odometer-tank fill method of MPG tracking.

But at least you know now that the meter is higher than it should be so you have a higher "target" MPG to aim for and stay at. This is also why I keep a little sheet with estimated miles at 1/4, 1/2 and 3/4 empty on my tank and their respective MPG values. That way I can see if my scangauge is close or off from what my fuel gauge/odometer is telling me where I should be at.

For example, with a half empty tank (5.5gal), I expect at least 220 miles on the odometer for a ratio of 40 MPG/tank.

[Markmysite]

I've tracked mileage by log against my other car and it is off a little (like 1-2mpg)... but nothing like you are seeing at 5.5mpg. Weird

[swng]

My MB E320 also has a meter that is slightly too optimistic. The difference is much smaller than 5.5 mpg though.
I wish my Toyota Matrix also had a meter but it does not, which is bad.