Speedometer error going from 275/55R20 to 33s

Starting high means a small change

Most 33-inch upgrades begin from a tire around 31.6 in and produce four or five percent. This one begins from 31.91 in, so the step to 33.00 in is only 1.09 in and the error is 3.42%. Anyone arriving at this page expecting the figure quoted for 17-inch trucks will find it noticeably smaller, and the reason is entirely in the starting size rather than in the 33.

An indicated 70 mph is 72.4 mph. Across 100 counted miles, 103.4 have been driven.

The 33 on a 20-inch wheel is an unusual tire

A 33x12.50R20 has to fit 33.00 in of diameter around a 20-inch rim, which leaves 6.50 in of sidewall — an aspect ratio of 52, low for a flotation size and lower than most 33s. Compare the same tire on a 17-inch wheel and the sidewall is 8.00 in. The name is identical; the casing is not.

The width is the real change here

10.83 in to 12.50 in is 1.67 in of extra section, which is a far bigger proportional change than the 3.42% of height. A 275/55R20 is a narrow tire for a 20-inch wheel and a 33x12.50R20 is not, so the consequences of this particular swap land in clearance rather than in calibration.

As a rotation count

632.0 revolutions per mile before, 611.2 after — about twenty-one fewer turns for every mile. That count is what the vehicle actually measures, and 3.42% is only a more readable form of the same fact. It is also the figure that would multiply an axle ratio if the gearing were being brought back to where it started.

Half an inch at the axle

0.55 in of ride height, the other 0.55 in going upward into the arch. Together with the width, that is the whole geometric description of the change. Load the pair into the calculator for the full speed table, or use the 275/60R20 route to see what nearly the same height increase looks like without any change in width at all.