Checked your odometer, lately?

[damo]

I checked out my odometer tonight over 20 miles using gps. It underreads by 1.5 to 2%. Well check again over a larger distance.

So if you can it's good to check it out to make your mpg figures even more accurate.

[Froggypwns]

My GPS is spot on with my car at highway speeds, and all my current bikes read a few MPH high. My past Japanese bikes read a good 10-15mph higher than the GPS on the highway.

[MetroMPG]

For those without access to GPS (or mile markers), remember you can also calibrate your odometer using Google Earth:

Using Google Earth to measure odometer accuracy

[pstrbrc]

Originally Posted by Piwoslaw

I have to set the cruise control to 73 km/h for the GPS to show 70 km/h (more than 4% off), but at 100 km/h the cc also has to be 3 km/h more (103 km/h, only 3% off). It appears that the difference depends on speed?

Actually, it's not unusual for a speedometer to be +/- 5%, especially if it's a mechanical "eddy current" type. For FE, the question is, is the odometer accurate? If you have a mechanical speedometer/odometer, there is a direct mechanical relationship between the number of times the transmission output shaft rotates and the number of times the odometer digit is advanced. Using a mileage check with highway mile markers will tell you how close your's is, but this will always be "off" a little, based on the fact that there are only so many combinations of teeth in the possible gear sets available, and the fact that tires are never "exactly" 25.3" in diameter (or whatever the nominal diameter is for the tire on your car.)
The latest speedometers are digital, and "count" the number of pulses from the car's VSS (electronic sensor, usually attached to the transmission's output shaft as well, but on some of the new cars, counting pulses off of a front brake's anti-lock sensor) that are calculated to make up a mile. Obviously, you can get a lot closer with these, since even if the VSS sends only one pulse per output revolution, if you have a 3.9 final drive and 25.3" diameter tires, and the speedometer has to round off to the nearest pulse, it could only be something like 10" off every mile at the most. Of course, then there's tire wear.
But most aftermarket electronic speedometers allow you to calibrate the speedometer as often as you like.
But if you DO have a mechanical speedometer, just check the accuracy of the odometer against a verified distance (interstate mile markers are handy, but measuring 10 miles is 100 times better than one), calculate the odometer error, then correct your logged mileage numbers accordingly.

[NeilBlanchard]

Hi,

I rechecked my xA today, now that my tires are getting close to worn out -- they are on their last season, and the under-recording has dropped from 3.5% to 2.3%. So, from now on until I switch to my snows, I'll be using the lower correction figure on my FE log.

[Edit: I used mile markers on Route 24 this time -- the actual error was a 0.6 mile loss in 26 miles.]

[Old Tele man]

...first thing I did when we got our new '09 Vibe was have the speedometer calibrated, and it was found to be "off" by a consistent 2mph LOW--actual 48mph, speedo 50mph--all the way from 20mph thru 80mph. But, since the speedometer is actually a stepper-motor driven from the computer, it seems like the computer is programmed to "add" 2mph to whatever input comes from the wheel rotation sensor.

...however, that raises a question about "what's happening?" with the odometer...is it being incremented from the "direct" signal (usually 1,000 pulses per mile) or is it ALSO being "tweeked" by the computer? Best I can tell using the Interstate mile markers is that it's responding to the "direct" signal, but short of getting a GPS unit, how would one check this with certainty?

...I ask because the Arizona Department of Transportation (ADOT) says the "mile" markers are NOT guaranteed to be exactly a mile apart, only approximately so, within the error limits of the calibrated "5th-wheel" that they use, and the fact that mile posts are placed equally on either side of the roadway, so lots of "one-sided" bends/curves in the road can slowly accumulate into a noticable error. I asked about divided highways, and was told they only measure via one side, but didn't say which.

[dcb]

mile markers are probably more accurate than gps, especially when sample size is large. Nobody needs to run out and buy a gps just to check their odometer (or their speedometer).

[jonathan150cc]

In my experience those marker signs are not 100% spot-on in their positioning either. They are just guide posts; Not GPS-calibrated-dowwn-to-the-millimeter-markers.

[Sean T.]

I just upgraded to 16" wheels (but don't worry! They're still lighter than my previous 15" wheels) so my odometer reads off.

I hadn't checked it when I had the 15" wheels, but now for every 18.9 miles my odometer reads, I've actually driven 20 miles!

So that means I get to multiply my FE by about 1.06 which is a pleasant surprise when I go from what the scangauge has been telling me vs. what I end up putting into my fuel log.

[extragoode]

How do you get the odometer calibrated? Can the SG do this? I've changed my tire size and doing the math each time fill up is a pain, granted it's only about every month. I don't trust the mile marker signs, I've used Google maps distance calculator and GPS. I put the next size up tires on our jeep when we got new ones (245/65/R17s as opposed to 235/65/R17s) because that's what I could find in good used condition ($140 installed as opposed to $325 installed new and they're the same brand as stock!) and our variance to the GPS went from 2mph over @70mph to 1mph over (speedometer showed 72mph @ 70mph on the gps, switched to 69mph with the new tires.

Sorry if that's hard to read. I really need to work on being clear and concise.