Tire revs/mile and odometer calibration

[Focus-Ak]

Rounding error needs to considered for an odometer or the Ultra-Gauge (not sure about Scan-Gauge; haven't used one), because a digital readout has a precision of only 1/10th of a mile (i.e. one digit). For calibrating distance U-G recommends using an accurately measured mile. However, a reading of 1.0 miles on U-G readout could be anywhere between 0.95 to 1.14 miles, assuming the U-G uses standard round practice. This means that when using this method (1 mile distance) the precision has a broad range of -5% to +4%, even if the test distance is precisely measured and marked (google earth or GPS). Calibrated this way, for an indicated 40 MPG, the true MPG could be anywhere between 38 and 41.6 MPG. It may be possible to reduce the uncertainty range by carefully watching for the OD to just click over to a new tenth, then start the calibration drive and carefully watch for the next mile to just click over. However, this means that the starting and ending points are at arbitrary (not predetermined) locations, meaning that using a GPS and traveling slowly would be necessary.

While a longer test distance would seem to result in a more accurate calibration because of less possible rounding error, the question then becomes how precisely can a longer, actually traveled test distance (say 10 or 100 miles) be measured? That precision would be harder to verify than a one-mile distance.

[Chrysler kid]

My hx has an s40 LSD transmission, using a live data scanner my vss reads 2-3 mpg slower than the odometer on the dash, it also show engine revolutions 2-300 rpms under what the need indicates.

If my gauges are not accurately displaying data then my odometer and my trip odometer are reading incorrectly as they are likely mechanical.

Guess I'll need to download that gps app to track my true distance traveled

[Fat Charlie]

Focus-Ak- So you eyeball the car's odometer against the measured mile. Yes, waiting for the "click" can give you somewhat arbitrary start and end points, but you can line everything up and in the worst case you can measure the offset at the start and end points.

Then you calibrate the aftermarket gauge to the odometer over the course of a few tankfuls. Being a couple tenths off after a few hundred miles is accurate enough in my book. Keep checking it, though- I bump the numbers every several tanks when I've been able to use the same pump a few times in a row.

[California98Civic]

The main readout of the UltraGauge rounds the distance reading to the nearest tenth, but in the calibration page you get a finer grained reading, to the hundredth or thousandth, IIRC.

[mcrews]

I may have mentioned this before....
When calibrating the SCANGAUGEll, I change the setting from miles to kilometers, same on the gps that I'm calibrating it with. Gives a more exact reading.

[gone-ot]

+1 on metric vs. foot-lbs-per-fortnight.

[Fat Charlie]

We're measuring distance here. I use furlongs.

[California98Civic]

Through repeated tests with a GPS app and google maps over significant distances, I find my new 15" wheels with 175/70-R15 tires on gives a 9% undercount on the odometer. I tested a defined section of the route to El Mirage from my home. Measuring 100.7 miles on the OEM odometer, the GPS showed 110.76 miles. I'm gonna call it 9% to be conservative. Speedometer is another story. Very odd since speed and distance use the same sensor mounted on the transmission.

[D.O.G.]

Originally Posted by California98Civic

Speedometer is another story. Very odd since speed and distance use the same sensor mounted on the transmission.

I think speedo / odo differences are quite common, it's just that most people don't check both.

Australian design rules require speedo accuracy to be between 0% and 10% fast, if your speedo says 100kph, you're really doing 90-something kph. I guess the U.S. rules would be similar.
On the other hand, I'm not aware of any restriction on odometer accuracy.

My Mazda and Toyota both have non-standard wheel /tyre sizes.
The speedos on both are accurate to my GPS, but the Mazda odo reads 5% slow and the Toyota reads about the same % fast.