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.

[lunarhighway]

are you still running the original tire size?.... although that would also offset the odometer so you'de get the same error margin on your manual FE calculations.
or maybe:
some speedometers are reading out faster speeds than you're actially going too, for whatever reason (keep you from speeding... make the car seem faster than it really is?)
but if this data is used to compute your FE this could also account for the difference.

if the odometer is correct than you'd indeed see a difference

just thinking out loud

[Yahma]

Originally Posted by 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.

According to the test done in Prius vs. Diesel Jeep Patriot by the auto-magazine 5th Gear, the Toyota Prius' trip computer was a whopping 17.1 mpg too optimistic.

The trip computer on my BMW 325i seems to be spot on based upon my odometer-tank MPG tracking.

[Bob64]

the problem with that comparison is that they didn't factor in the prius's variable sized gas tank bladder...

[nitrogen]

I have a scanguage, and have run the first calibration for it.

My wife is currently driving the car, and made me remove the guage because it "bothered" her, so I'll have to redo everything once I get my car back.

Hopefully the SG does a better job.

I love this thing though, according to my SG, I was getting 25mpg or so by just doing some basic dwb and dwl -ing.

[LLStimp]

My 2005 Sebring Touring convertible 2.7 L V-6 has the built-in gauge for fuel economy and it is off about 7% on average. Always optimistic, it's best guess was 3.8% over, it's worst error was 10.7% high. That's based on calculating FE using the trip odometer and the gallons at the pump numbers.

I use it for a relative indication of instantaneous (every 3-4 seconds updated) MPG.

Wonder if there is a way to adjust it's calibration to get it closer to reality, tho it is so inconsistent that I'd still be working with very fuzzy numbers. But it is better than nothing, I'm sure.

[dremd]

I know that the Chrysler sebring Vert I had in Vegas last year was hopelessly optimistic.

Admittedly we beat the tar out of the thing; indicated mpg 21.x Actual mpg 12.3

I've heard several people refer to the for power stroke display as the factory lie-o-meter.
It is certainly the source of the " I drive an f250 4wd jacked up and I get 27mpg at 80mph" people who seamed so prevalent last summer.

[ankit]

Yup mine is optimistic also. Maybe it's a nissan/infinity thing. Mine is almost always 2 MPG off give or take a few tenths. So I know the ballpark area of waht I am getting. Do you reset your FE meter everytime you fill-up? I do this (hold my trip button down when it is on the FE screen) and it levels out at about 1/4th of a tank. Anything after that mark I can take as being 2 MPG optimistic.

[whitevette]

Hi, all !
MPG is a shifting beast. So many factors affect this number.
If a ScanGauge (or any other such device) is added to your vehicle to keep track of this beast...this does not mean you can put away your pencil and paper. The gauge needs to be checked after installation before you can put any faith in it. All that is required is to keep jotting down the number of gallons per refueling / along with the odometer reading, and as the miles build up...check to see how close (or far apart) the two MPG curves are to each other. In a "perfect world", the average curves should almost overlap. In our mass-produced world, there will be a spread. This takes no effort...just a resolution to do it. It is not even necessary to fill up each time you refuel. Just start with a full tank ; note the gallons
( nearest tenth) pumped... and remember to use the last data point at the "point + 1" note - to get the mileage used correct.
Eventually, the total gallons used will approach a high accuracy ( for the odometer readings ( you did check the accuracy of your speedo via the mileage marker / time method, didn't you?).
If you really want to split hairs...the corrected odometer reading / gallons pumped method can be used as a "correct" baseline, and the calibration adjustments for the SG ( via the tire diameter / gas tank capacity pre-set parameters) can be adjusted ever so slightly to bring the digital ranges closer to the "correct" number. This is a pain, but it's better than not doing anything.

[Matt Herring]

My built-in 4runner mpg gauge always reads much more optomistic mpg than I actually record when I do the math to make an entry in my fuel log. Usually it's in the 2.5-3mpg range.

I have noticed that it is way more off on city driving than highway driving. On a recent highway only road trip my mpg gauge was only off by about .5 mpg. Usually on my 90% city/10% highway tanks it's off by as much as 3 mpg!

[trikkonceptz]

Also consider that some MPG meters, like some of the ones on Ford's are not true tank averages, but averages for a range. In the case of some Ford's the MPG meter is accurate to the last 50 miles driven.