How much speedometer error do 37s cause?

Past the point where it reads as a small correction

At an indicated 70 mph the vehicle is doing 79.2 mph. The gap is 9.2 mph, which is wider than the difference between most adjacent posted limits, and it is present at every speed in proportion: an indicated 30 is 33.9, an indicated 80 is 90.5. There is no speed at which 13.12% becomes negligible; it simply gets larger in absolute terms as the needle rises.

The diameter change behind it

285/70R17 is 32.71 in and a 37x12.50R17 is 37.00 in. That is 4.29 in of diameter, 2.15 in of it at the axle, and the largest single step most people make in one go. From a 265/70R17 the figure is larger again — 17.07%, putting an indicated 70 at 81.9 mph and turning 100 counted miles into 117.1. That starting size is 31.61 in, so the step is 5.39 in of diameter rather than 4.29, and the extra inch and a tenth is worth nearly four percentage points of error on its own.

The odometer is out by the same amount, permanently

113.1 miles for every 100 the display counts. Over a year that is a substantial divergence between the recorded and the real total, and unlike the speedometer it cannot be mentally corrected after the fact — the number in the instrument cluster is the number that gets written down.

What else is keyed to the rotation count

Wheel speed is not only a display. It feeds the transmission's shift scheduling, the cruise control's target, the anti-lock and stability systems' comparison between corners, and on many vehicles the tyre-pressure monitoring logic. A 13.12% shift in rotations per mile moves all of those away from the reference the calibration assumed. Correcting the displayed speed addresses the display.

Enter your own starting size — it changes the answer more than the 37 does — or see what the same change asks of the axle ratio.