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Speed calibration (speedometer)

I assume the speedometer is calibrated based on wheel rotation. Is it based on the front axle? (makes sense since they won't slip)

Does anyone know how to re-calibrate the car if you get different size tires on the speed-measured-axle (I guess Tesla would be the place to go)? Has anyone attempted to do this yet?

Down the road I may consider different wheels (currently have the standard 19's), and was wondering what effect it will have on the speedometer.
 

jerry33

(S85-3/2/13 traded in) X LR: F2611##-3/27/20
Supporting Member
Mar 8, 2012
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Texas
Down the road I may consider different wheels (currently have the standard 19's), and was wondering what effect it will have on the speedometer.

The wheels themselves don't have any bearing on the speedometer. It's the RPM of the tires that is important. You could have a 10" wheel (assuming it would fit over the brakes) :) and if the tire had the same RPMs as the OE tire the speedometer wouldn't change.
 
The effective outside diameter of the tires is the same regardless of whether the wheel is 19" or 21". This is achieved by having lower profile tires.

Yes I get that.

If you put 265/40 (20" wheel) on the rear, it will change the diameter (and therefore RPM). There isn't an aspect ratio for a 265 that will closely match the 245. (A 245/40R20, 245/45R19 and 245/35R21 are very close in RPM, but a 265/XXR20 will be bigger or smaller by a fair margin depending on what you choose for XX)
 
Question still not answered. Allow me to ask it a different way. If I install tires that are about 40mm taller, the speedometer and odometer are now going to read low. The computer systems make decisions based on the standard tire height / revolutions per mile.

So after installing taller tires, is there any way to recalibrate the system for the different height tires?
 

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