Speedometer Calibration

[MidwestOffroad]

GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off.

Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on.

[bigcountrysg]

You still have to factor in that the gps maybe off. Being you are running stock tires, stock gearing and no work has been done to your tranny as in a rebuild or replacement. There for your speedo should be accurate and not needing calibrated.

[MidwestOffroad]

a 265 to a 285 tire change is equal to a 3.7percent change in the speedo so at 60 your actually running 62.2mph


a 265 tire (31.6) will run aprox Auto 2443 rpms and in OD 1666
a 285 tire (32.8) will run aprox Auto 2354 rpms and in OD 1605

Yes this small change wont affect the engines vitals, but can make a difference in mileage

[MidwestOffroad]

the GPS are just as accurate as a Police Radar gun.

The factory speedos are as accurate as the person or machine sticking the needle on the gage cluster.

they put the needle on at a idle if it is off a 1/32 and your tires are undersized or over sized which you will never find a tire true to size. theres enough variables to throw the speedo off. but not enough to worry about with a diesel it can make a world of a difference in a 4.6 in a f150 though.

---

Last edited by MidwestOffroad : 04-12-2007 at 09:57 AM.

[bigcountrysg]

Originally Posted by MidwestOffroad

a 265 to a 285 tire change is equal to a 3.7percent change in the speedo so at 60 your actually running 62.2mph


a 265 tire (31.6) will run aprox Auto 2443 rpms and in OD 1666
a 285 tire (32.8) will run aprox Auto 2354 rpms and in OD 1605

Yes this small change wont affect the engines vitals, but can make a difference in mileage

That is if your tranny has the correct gear ratio for your speedo calibration as well. Myself running a 285 tire, with rebuilt tranny my speedo is off by 4 mph. So when I am running at 65 mph according to my speedo I am actually doing 69 mph. The shop that rebuilt my tranny told me they put a different od gear in my truck. To improve my gas mileage. If they did I have not noticed the improved gas mileage.

While trying to calculate your speedo there are alot of factors you have to figure in when trying to do it yourself. The best thing to do is go to a shop and have them put your truck on a calibrated speed wheel. It looks like a dyno table. But it measures how fast your tires are spinning the wheel. In the long run it costs about 1,000 to do this. I had it done once, with my 86 t-bird my speedo was off by 10 mph in that car. This speed wheel is so accurate. That there is no factor for error. That is why it costs 1,000 to have your vehicle put on it.

[redwoody]

Originally Posted by bigcountrysg

That is if your tranny has the correct gear ratio for your speedo calibration as well. Myself running a 285 tire, with rebuilt tranny my speedo is off by 4 mph. So when I am running at 65 mph according to my speedo I am actually doing 69 mph. The shop that rebuilt my tranny told me they put a different od gear in my truck. To improve my gas mileage. If they did I have not noticed the improved gas mileage.

While trying to calculate your speedo there are alot of factors you have to figure in when trying to do it yourself. The best thing to do is go to a shop and have them put your truck on a calibrated speed wheel. It looks like a dyno table. But it measures how fast your tires are spinning the wheel. In the long run it costs about 1,000 to do this. I had it done once, with my 86 t-bird my speedo was off by 10 mph in that car. This speed wheel is so accurate. That there is no factor for error. That is why it costs 1,000 to have your vehicle put on it.

back to the original question. I know I am 5% off because I used 3 different autonomous GPS (not survey grade) consumer/recreation-grade recievers.

1) a Garmin 12CX
2) a Garmin Rhino 120
3) a Magellan (not sure model)


all 3 were well within 0.5 mph of each other. We tested it over 2 days too (about 11 days apart) so we had a different constellation of sat's and diff atmospheric conditions. (we were traveling to Baja - long ways from Idaho). true, we likely had some multipath off the hood and windshield but I believe the ave of all 3 to be true.
my speedo says 80mph but I am really only traveling at 76mph. my tyres are stock 265-75 16".
5% off in the long run is a lot! that's 5000 freakin' miles in 100,000 miles! I had hoped to be able to tell the stealer and he could adjust accordingly. thanks for the dialog!