I would like to confirm the speedometer error on the GenIII vehicles both in the OEM 15" diameter wheels and the OEM 17" diameter wheels. There are many reports of the speedometer being in error by approx. 1-2mph too fast. I would like to confirm this using GPS data as well as ScangaugeII data if you have it. This will help inform tire and wheel upgrades for those who wish to get rid of the error. Methods: Simply find a stretch of road where you can maintain a constant speed of 50+ and it is safe to fiddle with your GPS. Use cruise control to maintain your current speed then use your GPS speed function or ScangaugeII to measure your speed against the vehicles speedometer reading. Report back in this thread with your vehicle Year, Model, Wheel and Tire Size as well as the make and model of your tire. Please include tire wear (brand new, 3/4 tread, 1/2 tread, bald, etc.). Also include information on the device you used to measure your speed. Thanks in advance for any help you can provide.
When my Gen III was nearly new (under 1,000 miles) I took it out on it's stock 15 inch wheels and tires with my Scangauge II and a Delorme Earthmate PN-20 GPS. GPS and Scangauge read the same at around 55 mph and the factory speedometer read 1 mph fast.
I checked mine when it was new, 17" wheels and Toyo tires. Error was 2 mph at 70 and about 3 mph at 100. I just had new Michelins installed and I'll try to check them today. It may be of interest that the speed on the ODB per Scan Gauge is right on, same as the GPS. So the car "knows" how fast it is going and the speedometer error is by design to comply with the U.N. regulation.
Thank you guys for the input. It was brought to my attention that the 15" tire size and the 17" tire size are different enough that the 215/45/17 will turn more revolutions per mile thus making the speedometer read a bit higher than the 195/65/15. This could be why the speedo reads higher for Tumbleweed than for xs650 and why the UK mpg difference test only produced a 2mpg difference. I do not think the tire circumference difference is that great but I'm not an expert so who knows. I should also add that I have observed a consistent error of 2mph (too fast) in my GenII (with both my 185/65/15s and my 215/45/17s. Other GenII owners have observed the same error.
The difference in factory 15 inch and 17 inch revs/mile is small. Considering that the speedometer resolution is 1 mph, Tumbleweed and I might be as well be observing the same car because of rounding errors from the speedometer readout alone. A speedometer that reads 2.5% high would show the same errors we both saw when rounded to the nearest whole mph. Speedometer error is normally much closer to being a percentage of speed rather than a fixed constant error.
i was thinking about this the other week while on a long distance drive... i do drive a genII.. so... it's not a genIII but i see problems in the speedo and such. I use the little button in the mfd that says calibrate wheels for speedometer.. or.. whatever it is.. and i noticed that after using that.. my gps speed is about the same as my speedometer... however.. i noticed my Odometer is off... by about that 2 to 3 percent in which the speedo was usually off.. it's clocking less than i actually drive. if this has been happening all along.. i assume i have an extra 3 to 5 thousand miles on my odometer... er. car.... which is missing from the odometer. (then again.. i do have aftermarket wheels) so.. i'm more or less wondering if anyone else has seen a speedometer and odometer difference...
Re checked today with my new Michelin tires (200 mi) and the error still seems to be about 3% maybe a bit less. I check it by putting my GPS right up next to the speedometer display and looking at both when the speedometer clicks over to the next higher number, i.e. from 49 to 50mph. I checked at 50, 60, 70, and 75 mph and the percent of error seems consistent. I think my odometer is a lot closer than 3%, maybe about 1% high, but I need to look at it again over a longer distance.
My speedometer reads consistently 1mph higher than my GPS at highway speeds. The tires are OEM 15" Ecopia's.
Speedometer and odometer calibration are unrelated. The odometer calibration is fixed by law, to keep people from tampering with mileage. Unfortunately for you, your non-standard tire circumference throws off the fixed calibration. Tom
Tires, Michelin Energy Saver, 195x65x15. Speedometer reads 1 mph in the postive. i.e. 55 mph indicated 54 mph true