Speedometer recalibration with wheel size change?

[BocaDave]

Does the dealer (or wheel installer) recalibrate your speedometer to account for a larger wheel diameter when you upgrade to larger wheels?

If not, of course, your speedometer would not read accurately with the larger wheels (or smaller, for that matter).

The difference, if I remember my high school math, will be proportional to the square of the radius.

So... going from 16 inch wheels to 18 inches would be a 25% increase in tire circumference, and presumably a similar error in speedometer speed.

How do they adjust for this?

 

[Disaster]

The size you are talking about is just the rim size. The tire size is what really matters. When going to larger rims, manufacturers use lower profile tires. The smaller the number, the smaller the profile (height over width.) For example, the 17" tires are 55 series while the 18" are 50 series. The rolling diameters are usually with a couple percent difference. Vehicle manufacturers don't vary it more than that so they only have to release one speedo gear...or calibration.

By the way, it isn't always perfect anyway. There is usually a range where the speedometer is the most accurate...around 60-70mph. At very low or very high speeds there is usually more error (though modern electronic digital speedos could compensate for this.)

 

[Rey]

Recall that TireRack has data on tire diameters.
If you really want to know true speed the best way to determine, in my opinion, is via a GPS that has a speed function.

 

[crepen]

Your high-school math memory is failing.

The dimension of interest to the speedometer is circumference, which is 2 x Pi x r, so it is proportional to radius, not the square.

regards,
cmr