Odometer Error = Lower Real MPG
 

Sorry folks, your HCH2 isn't going as far on a tank as you think.

I had the oppurtunity to check the odometer accuracy yesterday on my 06 HCH using a Garmin handheld GPS. The car indicates it has traveled farther than it actually has which means when you divide tank miles by gallons, your MPG is actually lower than it really is.

The error in my case is to the tune of for every 45.5 miles I drive, I've actually only driven 44.5 (about 97.8%). Another way to look at is, if you get 600 indicated miles on a tank of 12 gallons you would compute the MPG to be 50 miles/gallon. In reality it is 48.9 MPG.

My testing methodology went like this:
1. stop the car
2. reset the trip B and the garmin GPS
3. drive (mostly freeway, 24 miles in my case)
4. pull on to side street
5. stop the car as soon as the car odo ticks over to the next tenth.
6. Record the numbers

If I were to guess, part of the error comes from running the tires over the recommended pressure (only 40 PSI in my case). I suspect this slightly increases the circumference of the tire and accounts for all or some of the error.

Has anyone else checked their odo?

-dan

Re: Odometer Error = Lower Real MPG
 

I'm not throwing up a BS flag, just trying to add some constructive test criticism in here. A more accurate test would be a completely straight course without any turns. Depending upon the sampling rate at which the GPS handheld receives updates from the satellites, the handheld could be rounding off corners instead of recording the whole distance. If you feel frisky, try it again on about 10 miles of straight freeway. This will reduce any possible error from the measuring device.

Re: Odometer Error = Lower Real MPG
 

I also have a heard time understanding how higher tire pressure and larger circumference could lead to displayed distance being shorter than actual.

I would also go with the GPS error theory, though I think GPS could just as well be overestimating as underestimating the distance.

Best way to check this is to compare with the road signs, if you take a longer trip some day.

Re: Odometer Error = Lower Real MPG
 

Wouldn't overinflation increase the tire's circumference? If so, the car would travel further for each wheel rotation. Therefore, the distance shown on the trip meter would be less than that actually travelled. That's opposite of what you're saying.

Perhaps there are other reasons for the difference? A higher-mileage HCHII with lots of tire wear might have worn enough rubber to change the circumference to be a few % smaller... or the GPS may have gotten a weak signal that day. I have a Garmin Quest (use it on my bike!) and sometimes the signal is off a bit when I'm riding in canyons and under lots of trees.

Re: Odometer Error = Lower Real MPG
 

Also think we need to have a much larger sample size than one (1) to draw any meaningful conclusions.

Re: Odometer Error = Lower Real MPG
 

Over inflation can cause many things, but there is not enough change in Tire Circumference to make a real difference. I also think more than one sample would be needed to establish a pattern/trend. I have had good results with GPS, so I would not really be inclined to think the GPS is rounding curves, but if a larger testing sample were available, that could be sorted out.

Interesting Stuff.

Bill

Re: Odometer Error = Lower Real MPG
 

I agree that driving as straight as possible would certainly remove some of the variables from the test. According to Garmin's site, the sampling rate of my unit is once per second which seems like it's not going to be clipping off big chunks of my route - certainly not to the tune of half a mile after 9 quarter turns (which is how many I made on the test). I would guess less than 150 ft of error from the turns, if that much.

Anahymbrid is dead right about the over-inflation and I stand corrected. Larger circumference = you've gone farther than than your odo reads which wasn't what I saw. So I agree over tire inflation is not the culprit and, if anything, it might make the error less severe.

So another theory, and a weak one, is that tire wear (only 5000 miles for me) has decreased the circumference.

A more likely theory, and I've heard car makers do this on purpose, is the speedometer just is calibrated to read slightly high. Honda wouldn't want to get sued by people who got tickets for exceeding 65 MPH when their speedo was indicating 65 MPH. So to protect themselves (and us), they make the speedometers (and accordingly, the odometers) read higher than actual.

It would be great to get a few more data points from other HCH owners to see if this affects the larger HCH population.

Maybe the good news about this is my 36,000 mile warranty should be extended by another 809 miles for me (and probably most of you). You just need to prove it when push-comes-to-shove.

cheers,
-dan

Re: Odometer Error = Lower Real MPG
 

Check your real time speed with the Garmin as compared to the speedo in the car, should tell you right off the bat if that is the culprit.

Bill

Re: Odometer Error = Lower Real MPG
 

If you Turn the Standard radio with the 1 and 6 buttons depressed, The radio enters daignostic mode,
Now press 5 and the readout will be the vehicle speed, turn radio off to return to normal


Interestingly this mode is sopt on with my GPS, and 3 Kilometers under the Speedo. Go Figure.

Re: Odometer Error = Lower Real MPG
 

Where the frack did you get that tidbit from, Steve? Is it something unique to Australia, navi/non-navi, or what?