I thought I put this to rest last week but after a 200 mile drive I've got it straight in my head (I think).

Using my Garmin iQueM5 finally on a trip I found that in 198 miles (by GPS reading) of driving my odometer read 195.1.

Also driving at 70 mph (by my speedometer) my Garmin was reading in the range of 68.5 mph.

That's why I think I confused myself last week. The errors are divergent.

The Speedometer reads too HIGH. To solve the speedometer error you would have to have a taller tire/wheel combo which would register a slower speed than the OEM's at the same speed, since it takes less rotations of the tire to correct the "too fast" speedometer reading.

However, since the odometer reads LOW you would need a shorter tire/wheel combo to increase the number of rotations per mile over the OEM's to correct the error.

The speedo error is reasonable and it keeps you under the speed limit and is a decently close margin of error. However the odometer error is factored into the FE readings and thus we probably are off by 1.5% (too high) on actual FE calculations. (Still it's pretty close too_not as bad as 5% as some have guessed at)

All in all both were pretty close. But a simple tire/wheel change can't solve both problems since the errors diverge as you make changes.

No responces necessary as I wore most of you out last week on this. I'm just clearing my head.

 

I would think that the GPS is less accurate than the odo. At best, a GPS is only accurate to within 9 feet unless you have the secret military codes????

 

My observations are the same as yours, which I've reported many times in here. My speedo appears to report 5% too high (reports 60mph when I'm really doing 57mph) and the odometer reports too low (which is why my FE calculations are 2% less than what the OBD is telling me). I'm more concerned with the speedo problem, since the highway patrol could care less what the odometer reading is!

Yes, but that is relative to the satellite at any given point. If you are off by 9ft from where you truly are "here", then you are also off by 9ft from where you are truly are 500 miles away. It's not like you get a 9ft deviation at every point of your journey. So at the extreme, the GPS odometer would be off by 18ft (you are the furthest away from true location at both ends of your journey) which is statistically inconsequential after 500 miles.

 

On a recent trip from Dallas to Colorado Springs, I used a newly purchased entry-level Magellan GPS. The map was accurate, but the altitude reading averaged 20% of the actual height - kind of humorous.

 

I'm more concerned with the speedo problem, since the highway patrol could care less what the odometer reading is!


Actually, you're going to have a hard time straight faced telling the cop who pulled you over for doing 75 "But my speedo said I was doing 80". The speedo error gives you a mild margin of comfort and is probably not much of an issue.

 

and the odometer reports too low (which is why my FE calculations are 2% less than what the OBD is telling me).

You and the OBD are using the same "wrong" odometer reading. It's not that much, maybe 0.3 mpg for the folks near the 39 mpg mark. On average over 12,500 miles I've found about 0.9 mpg error in the readings between the OBD and calculated actuals. The odometer reading won't affect that. The only thing you can do with the odo reading is simply use it as a correction factor after all calculations are made. Since it's only 1.5% I wouldn't worry about it.




However some have found their speedo to be off by 5% and if the odometer was 5% then that's a lot. However the errors in the speedo and odometer don't appear to be consistantly the same.


I also noticed difficulty comparing my speedo to the GPS. I would feel the car slightly slow or accelerate while the GPS rerading bounces around but the speedo would not move significantly. It appears to have a soft "dampened" or delayed operation. I assume since everything is electronic they don't want the needed vibrating like my 70's cars did.

 

Not really flop, i use a DeLorme GPS with my laptop, and I have always, and I mean always gotten a 100% accurate reading on the speed, yes the location may be deviated, but the relative speed is not, since speed is constant over time. I have actually found it to match within .2 MPH of my Digital display on another vehicle.

 

WvGasGuy, I can concur on the Speedometer error. On our trip, I used my Magellan Meridian marine portable GPS on the dash and noted that the speedo was off about 3 mph at most speeds. Cruise set exactly on 75 yielded 72mph according to the handheld GPS.

 

The speedometer value is intentionally biased a few mph to meet the ECE regulation No.39.
http://www.unece.org/trans/main/wp29...egs/39rv1e.pdf

Ken@Japan

 

Interesting info. One way to be absolutely sure of mph/distance would be to survey out a given distance using a calibrated GPS receiver in survey mode with mm accuracy then precisely time the route.

All we have now is an uncalibrated vehicle speedometer and an uncalibrated GPS so there will be undocumented built in system errors.