255/60R21 vs 235/60R21
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/60R21 | 235/60R21 | Change |
|---|---|---|---|
| Overall height | 33.0" | 32.1" | -2.9 % |
| Sidewall | 6.02" | 5.55" | -7.8 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 103.8" | 100.9" | -2.9 % |
| Revs per mile | 610 | 628 | +2.9 % |
| Axle height | 16.52" | 16.05" | -12.0 mm |
| Approved rim width | 7–9.5" | 6.5–8.5" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 255/60R21. Fit 235/60R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,714 on the road, so it over-counts by 286. At a true 70 mph the first turns 712 times a minute, the second 733.