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I noticed that my speedometer consistently reads +2 mph. Odometer seems correct, so this seems like an offset that is programmed in. If I set the cruise control to 80, my Garmin (using GPS) reads 78. It isn't proportional; it's 2 mph at all the speeds I checked.
I took a look around in my pseudo-DAS (Autel MP808) and while I found an odometer reset, I did not find anything for speedometer offset. Y'all with proper DAS, do you know if there is such an adjustment?
295/40R21: speedometer is +2-3 kph over Garmin GPS across the speed range 0-110 kph. Tires are 75-80% worn. Pirelli PZero. Will try to remember to post with new Geolandar tires when I get them (this summer, or next).
255/55R19: speedometer is +5 kph over Garmin GPS, from 0kph to 80kph, then it increases slightly and seemingly gradually and @ 110 kph it is +7 kph over Garmin GPS.
Tires are practically new, about 2000 km on Nokian Hakka 9 SUV not studded.
90 kph is the speed limit around here, so I don't "move the cob webs" a lot over that.
A friendly hello to the traffic police if they're stalking my online presence. Hi police officer!!!
I swear my new snow tires and worn trail tires have visibly different diameters. I can measure the circumference and check.
What you're saying is that the speedo clearly does not calibrate to GPS, at least in Canada, and what's more it appears to be some bizarre hybrid between proportional and fixed.
Your tire diameters are almost identical between the two, not counting wear of course.
It does not on mine, not that I can say with any certainty. I do find it interesting that for a range of 80k kph it is the same on one tire, and then slightly increases, again, seemingly progressively. But on the other tire it seems constant. The next time I am on a road with some space, I'll "measure" the 295s once again since I have them on for the summer. Not sure if tire pressure would make a difference. The 295s are 35 psi front and 38 psi rear. IIRC, the Hakkas were 32 or 35 psi all around.
Edit: my summer/winter tire diameters are different. The 295s have a notably bigger diameter than the 255s. Correction: the winter 255 tires have a noticeably bigger diameter than my summer 295 tires. I can see the difference when I stand the two tires side by side, or lay one over the other.
Specs from Tirerack for the 295s Iwhen new): total diameter 30.5"
Specks from Nokian's site for the 255s (when new): 31.1
So the summer tires are even a smaller overall diameter than the winter tires, considering wear. If my wear guesstimate is correct AT 80%, the PZeros are currently with diameter 30.2", assuming they started at 12/32 and are now down to 4/32.
Last edited by expl0rer; Apr 13, 2021 at 07:21 PM.
Yeah, I went out and checked. At lower speeds, the offset is smaller than at higher. So I guess my worn summer tires must be within the same rounding error (w/in 1 mph at highway speeds) as the new snow tires.
Too bad. I was hoping the computer actually put the GPS to good use. I guess not.
Around 40 years ago BMW produced a service bulletin describing speedometer offset.
basically, it’s not allowed for a speedo to display less than actual ... and although different model and different brand tires may be marked the same size, they aren’t necessarily equal. Therefore they deliberately read fast ... in that bulletin BMW stated +10% was acceptable. Bulletin noted odometer is accurate.
295/40R21: speedometer is +2-3 kph over Garmin GPS across the speed range 0-110 kph. Tires are 75-80% worn. Pirelli PZero. Will try to remember to post with new Geolandar tires when I get them (this summer, or next).
255/55R19: speedometer is +5 kph over Garmin GPS, from 0kph to 80kph, then it increases slightly and seemingly gradually and @ 110 kph it is +7 kph over Garmin GPS.
Tires are practically new, about 2000 km on Nokian Hakka 9 SUV not studded.
90 kph is the speed limit around here, so I don't "move the cob webs" a lot over that.
A friendly hello to the traffic police if they're stalking my online presence. Hi police officer!!!
Hello. I don't actually pull over speeders. Thats the traffic unit only. Unless you're shooting at me or someone else.. I assure you I won't be stopping you. Carry on.
Hello. I don't actually pull over speeders. Thats the traffic unit only. Unless you're shooting at me or someone else.. I assure you I won't be stopping you. Carry on.
I'd guess the traffic guys wouldn't bat an eyelash for 12-13 mph over on highways. The local boys don't usually bother for anything up to 20 kph on the highway.
Around 40 years ago BMW produced a service bulletin describing speedometer offset.
basically, it’s not allowed for a speedo to display less than actual ... and although different model and different brand tires may be marked the same size, they aren’t necessarily equal. Therefore they deliberately read fast ... in that bulletin BMW stated +10% was acceptable. Bulletin noted odometer is accurate.
Caution on the side of consumer benefit. I like that the speedometer is slightly over the GPS. Leaves me a margin of error. Not a big one, but nonetheless the margin is there.
Hello. I don't actually pull over speeders. Thats the traffic unit only. Unless you're shooting at me or someone else.. I assure you I won't be stopping you. Carry on.
Plus you probably aren't venturing into Canada to enforce their speed laws ...
Concerning the speedometer difference versus odometer. I took a 1102 mile (per Bing maps) trip, which the odometer (trip computer) recorded as 1117 miles. Difference = 15 miles, 1.4% over.
At speed, I was consistently seeing 2 mph (unfortunately, no decimal) over GPS, at speeds ~80 mph. That is 2.5% over.
So it appears the speedometer fudges slightly high over the odometer, which is itself slightly over actual (assuming the maps software is correct).
What a pity the system doesn't recalibrate live.
It's a hell of a road trip car, tho. Although these calculations imply that the actual gas mileage at 77 mph average is more like 18.4.
I'd guess the traffic guys wouldn't bat an eyelash for 12-13 mph over on highways. The local boys don't usually bother for anything up to 20 kph on the highway.
15mph over on highways and 11mph over on city streets.
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