2009 Dodge Ram 1500: Liar

I've noticed that the one in my WRX is about 1.5 miles optimistic as well.

Seriously, have you ever seen a honest trip computer?

They're usually off by 2mpg or more... My Scion XD's trip computer has been lying for its whole life, sometimes off by even 3mpg.

Your RAM is only off by 1.5mpg---its 50% more trustworthy than my Scion.

Chris, is your camera by any chance a Nikon Coolpix? Because I have one and it takes equally crappy pictures (no offense). I have yet to upload a picture that doesn't look grainy and fully in focus, all coming from a 12 megapixel, expensive camera.

Damn you Ashton Kutcher for selling me on a Nikon...never again.

I meant out-of-focus...you get the idea

rsholland: my WRX is optimistic too. it once said I had achieved 19.2-19.5 MPG average for a tank of gas but when I filled up and ran the numbers, it was closer to 17.7. It was in mostly city though with maybe 20 minutes of highway mixed in, so it was expected. I would have been excited though if I had actually gotten 19ish MPG with mostly city driving though, haha.

I don't expect these MPG "meters" to be totally accurate though, but they're nice to have just for ballpark figures.

Is it possible that it could be the other way around? I'm not sure how the on board computer measures mpg but it seems like your "manual" way seems to rely a lot on things you don't have control over: ie the gas pumps, and the assumption that when it clicks off full it does it in the same place every time.

Yes, but if you do enough of the manual averages then you should start to see a fairly consistent pattern that account for that variable (when the pump clicks off). If your formula is to take miles travelled and divide by gallons used, and most of the pumps click off in relatively the same place, then the manual method probably ends up being more accurate.

"Seriously, have you ever seen a honest trip computer?"

I have..... the one in my '06 Frontier was accurate to within a tenth or two. I checked it the old manual way for about the first 6 to 8 months of ownership and found it very close each time. I then took what it said as accurate - passing later spot checks as well.

As every good economist knows, you should never implicitly trust a government number. They're always a little...round.

irba: "They're usually off by 2mpg or more... "

Sadly, I agree. At some point, there will be an investigation and many manufacturers will be in trouble.

@zoomzoom22,

I'm definite no fan of their Coolpix line, but their DSLR cameras and lenses are very good. For compacts, I'll stick with Canon or Panasonic.

Back on topic, the average fuel rate calculation in my '04 A4 is usually 1-2 MPG off. I'll stick with the miles driven divided by gallons to full to get a better estimate.

The fuel economy calculation in our Taurus wagon is actually pessimistic by a couple miles per gallon.

Well going from 13 to 15mpg makes you feel good!
Truth hurts! LOL

The fuel consumption calculations are 4-6% optimistic on my X3 and MS3.

@alex4515: yes but I still don't see the belief in this post that the "manual" way is more accurate than the computer way. Assuming the computer measures the flow of gasoline into the engine I would consider that more accurate than using the gas pump and calculating averages which involves rounding and measurement errors.

Why 89 octane?

It took 16 posts but someone noticed the 89 octane.

Does the Hemi really need 89 or has the Corvette got everyone scared of the cheap stuff?

I've manually tracked my fuel economy since I got my car, it was because of another similar discussion about the trip computer did I start tracking the trip computer estimates, been doing that the past year. My '07 Mazda 3 is actually slight pessimistic, but it's consistent, no matter how high (30) or low (19) the MPG was for the tank, it'll be 1.1 to 1.5 lower than my manual calculations. For that reason, if you really care, you should note the trip computer and then do your own calculations, they're not mutually exclusive.

My 2009 Subaru Legacy averages 5% more optimistic estimates then hand calculated. But, it also only calculates at intervals of .3 MPG. So, it'll show 27.3, 27.6, 27.9, 28.2, etc. I am curious as to which engineer or programer made that decision. It can't be any harder for the system to display actual (not that it is crutial to display a more accurate 'estimate', but just why program it that way period).

@smrty - assuming that the gas pumps are normally clicking off right around the same point, then that is a relatively accurate guage of how much fuel you are using between fill-ups. I'm not sure what information the computer uses to determine approximate mileage - but if it's based on injector information, then a very small underestimation per pulse would add up to a pretty sizeable difference after a full tank.

Edmunds staff:

Have you guys had this problem with your '09 Fit? The gauge was so far off on those that Honda issued a TSB that has them do a re-flash of the computer to make it more accurate. It was routinely 10-15% off, which if you get 35 mpg can make it around 5 mpg off.

For the first many months I owned my Fit, the gauge routinely read around 40 mpg, sometimes even 41 or 42, while the calculated mileage I was getting was more around 35-38. Now after the re-flash, it's down to only being about 1-2 mpg off, though it's still always high compared to my calculations--it does not fluctuate where one tank it will be high and another will be low.

@ranger-bob
My 2009 Tribeca does the same thing, however the estimate is within .2MPG of the actual, so it really doesn't bother me. They probably started the .3 interval in 2006 when their first nav. debuted on the B9 Tribeca.

The HEMI performs optimally on 89. It can run 87 with no harm. I have run 87 in mine since purchase. If you want to see really crummy economy numbers and performance, put an ethanol blend in a HEMI.

I don't think I've ever seen a car that asks for 89 octane. What a weirdo.

Mazda 12A and 13B rotary engines call for 89 octane.