235/65R21 vs 255/60R21
Going from one to the other changes the overall height by +0.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/65R21 | 255/60R21 | Change |
|---|---|---|---|
| Overall height | 33.0" | 33.0" | +0.1 % |
| Sidewall | 6.01" | 6.02" | +0.2 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 103.8" | 103.8" | +0.1 % |
| Revs per mile | 611 | 610 | -0.1 % |
| Axle height | 16.51" | 16.52" | +0.3 mm |
| Approved rim width | 6.5–8.5" | 7–9.5" | 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 235/65R21. Fit 255/60R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | +0.0 |
| 50 mph | 50.0 mph | +0.0 |
| 70 mph | 70.0 mph | +0.0 |
| 90 mph | 90.1 mph | +0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,006 on the road, so it under-counts by 6. At a true 70 mph the first turns 712 times a minute, the second 712.