smileyw

I have a 6spd Navi, and use the trip computer a lot. I have noticed that the odometer and the trip computer are off from each other. It looks like the trip computer gets off about .1 miles for every 50 miles or so. (For example, right now the odometer says 410.9 miles, and the trip computer says 410.1.

Does anyone else have this problem, or is it just my car?

larchmont

Well, well, well. I thought I was as ..... uh ....... "precise" as anybody, but I'd never have thought of wondering about such a tiny difference.

Sounds pretty darn close to me! I think the difference is probably smaller than the basic accuracy of those gauges anyway. Things like tire pressure can affect measured mileage; whether that would affect the two gauges differently, I don't know.

fdl

Be sure the .8 mile variation is growing. It could just be that they tested the trip computer and reset it when the car had .8 miles on it, which would put the trip .8 miles behind the odometer. If you put another 50 or 100 miles and the difference doesnt increase...there's your answer.

smileyw

Yes, it grows, as you put more "miles" on it. Sorry for not being specific about that.

smileyw

larch:
I am tring to figure out if there is something wrong or not. Your point about differneces is fine, but why would one be different than that other? Also, when it shows the MPG at the end of the tank of gas, it is well off from what my math says (gallons pumped / miles driven).

larchmont

Yeah, you did make it clear that you were wondering mainly about that. And it sounds fine to me.

In saying this, I'm assuming a couple of things about what you mean that the displayed MPG is "well off" from your calculation -- I'm assuming that it means "not that much" (no offense of course). I'd say that if it's within about 10%, that's close enough, considering the variables -- the main one being that each fill-up is a little different. (Actually that's the only variable that I can think of, but maybe there are others.) BTW a number of people have posted elsewhere that their own MPG calculations differ from what the car says, and I've assumed it's only because of something like this.

Not that this would be worth doing, but I think that if you were to calculate MPG in your own way over the course of several consecutive fill-ups (which mostly gets rid of that variable), and compare it with what the Trip Computer calculates (by the way the latter would be a little bit of a complicated calculation if you wanted to really do it right, but you wouldn't have to) -- I think the results would match within a fraction of a percent. If not, then maybe something's wrong.

smileyw

Actually, "well off" was someplace around 2-3mpg on a 15 gallon fill up. So the trip computer would say 28 mpg, and my calculation is say 24-25.

I am mostly curious to see if anyone else has this same problem with the mileage being different?
Anyone looked?

larchmont

Bingo!

As I said, anything up to about 10% (which this is) could reflect just differing fill-ups, and I would guess it probably does.

idx

Same here - I check the mpg all the time. Odo is about .5 to .8 more than the trip on a consistent basis and I reset both after each refill.

As for MPG calc, I've seen the same exact thing with the exception of last night. The trip comp was being weird and registering 28.8 all the time. After the refill it was down to 28.5 or so and my manual calc actually came back with the same. Typically the comp will say 27 something, but I calc 25.5 or 26. (I have all the numbers in a book for each refill -- I'll post them sometime if you're interested.)

Several weeks ago I was thinking this might be the 'range buffer' in the Nav, but I'm not sure yet. ( http://www.acura-tsx.com/forums/show...ghlight=buffer ) I'll have to see how it acts after a few more fill ups.

-r

larchmont

Don't know about anyone else, but I sure would be. I think the most important thing would be to look at a number of consecutive fill-ups. If possible, it would be good for you to indicate which ones are consecutive. (From your post, it seems they would all be consecutive -- which of course they are if you never miss putting one down.)

idx

Yep, all consecutive fillups. Typically all are a large-ish fill and roughly the same mileage. (12-15gal)

Maybe I should get off my duff and write that mpg tracker/maint log web app I've been thinking about...

-r

tsos

The general consensus was that it seemed to be it that the computer is 10-20% optimistic.

After a year of experience, can we conclude that the the trip computer is accurate or not?

Occasionally, I heard someone mention that it gives the MPG for just some portion of the tank - is this right?

BTW, I would think that the variance in fill-ups would be under 1 gallon, or <10% for a 10 gallon fill-up.

Dan Martin

If anything I would say the navi computer is more accurate than the odo. The speedometer is pretty accurate but it seems to be around 0.5-1% fast with the stock tires. This would mean that the odo would probably be high by about the same amount.

tsos

Are you saying that the odometer and trip computer don't use the same information (tire revolutions) to determine distance?

The speedo requires a d(distance)/d(time)=velocity, or some coupling between cable rotation speed and the speedo needle.

I.e. the trip computer could integrate the speedo to get distance, but this seems to be unnecessary.

But, even if the navi can get the right distance, it still needs an accurate accounting of the amount of fuel added. Does it calculate it from the last and new location of the gas gauge needle? Do it assume that a fill-up is a complete top-off (and it knows the true tank volume)?

Dan Martin

Navi uses GPS info to determine your speed and distance. I believe it also uses the speedometer when it doesn't have a GPS signal for dead reckoning.

I don't know how the fuel system works. I would imagine it gets the same reading from the gas tank your gas gauge does.

SacQuacker

Could it be that the nav calculates number of miles driven via GPS rather than directly linking with the odometer? ..Or am I completely lost?

SacQuacker

The nav and the trip computer relying on different sources of data certainly could account for their different results.

tsos

New School, and way cool. Someone was thinking.

So if you're driving backwards does the navi know (e.g. it may have a receiver at the front and the back)?

And does it remember where you were pointed before you turned off the engine?

idx

After a year its still consistently high. The one thing I keep thinking about is if we're trying to compare apples to copterfishhandshakes.. (..or oranges)

My MPG calc is the basic, miles driven / gallons of gas filled. The screen says average MPG whereas I'm calculating the actual MPG. I always think about this when I use the nav and it tries to calculate the time left. Its always too long - I have to assume its basing the ETA on my average MPH.. Pretty inaccurate if my AvgMPH is 40, then I get on the highway and keep it at a steady 80. Perhaps the nav is taking mpg samples along the way and averaging them out?

??

-r

Dan Martin

Forwards or backwards you're still going somewhere so your mileage will go up.

VeniceBeachTSX

Doesn't anybody actually bother to read the owner's manual? The manual for the navi system explains this.

The navi system (including trip computer) gets all its information from the GPS/inertial system. There are some slight inaccuracies inherent in any navigation system because it measures a series of "points" and computes the distance from one point to the next. Going around a corner quickly, for example, it might "cut the corner" in the calculation. Usually these systems measure location at pretty small intervals, which means the inaccuracies will be small at normal vehicle speeds.

The odometer is influenced by tire issues. If your tires are a bit over or underinflated it'll show a difference. If you're using non-OEM sizes, odometer and speedomer will both be off by at least a little bit.

Trip computer is probably more reliable than the odometer.

idx

No, we got bored and decided to drive the car .

On average I'd say the speedo is ~ .4-.6 greater than the computer. Although I only check the comp after gas fill up (on the `previous` page) so perhaps its truncating the decimal. At any rate, its pretty close whereas the MPG isn't. ~2MPG difference doesn't seem very close to me, but I can see if we're mis-interpreting it.

-r

dmbfan36_23

Thank god this is "normal"... I've had my 2005 w/NAV TSX for almost a month and I keep noticing the difference in the trip computer vs trip odometer readings. I always reset my trip meter (Trip A) when I fill up with gas, and I have the nav set to reset the trip computer on fill up. The last couple fill ups I've checked that bother were reset to 0 at the same time and drove... currently, 335 miles into this tank there is a .5 mi difference... I've noticed as much as .8 at times, it is infact a function of how many miles you drive.. so after 10 miles there's no difference, but around 50 you start to see it...... I had figured the trip computer might be feeding off the GPS satellites but I wasn't quite sure...