235/40R21 vs 255/40R21
Going from one to the other changes the overall height by +2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/40R21 | 255/40R21 | Change |
|---|---|---|---|
| Overall height | 28.4" | 29.0" | +2.2 % |
| Sidewall | 3.70" | 4.02" | +8.5 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 89.2" | 91.2" | +2.2 % |
| Revs per mile | 710 | 695 | -2.2 % |
| Axle height | 14.20" | 14.52" | +8.0 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 235/40R21. Fit 255/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.6 mph | +1.6 |
| 90 mph | 92.0 mph | +2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,222 on the road, so it under-counts by 222. At a true 70 mph the first turns 828 times a minute, the second 810.