245/60R21 vs 255/60R21
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/60R21 | 255/60R21 | Change |
|---|---|---|---|
| Overall height | 32.6" | 33.0" | +1.5 % |
| Sidewall | 5.79" | 6.02" | +4.1 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 102.3" | 103.8" | +1.5 % |
| Revs per mile | 619 | 610 | -1.5 % |
| Axle height | 16.29" | 16.52" | +6.0 mm |
| Approved rim width | 7–9" | 7–9.5" | overlap 7–9" |
Both are approved on a rim 7" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/60R21. Fit 255/60R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,145 on the road, so it under-counts by 145. At a true 70 mph the first turns 722 times a minute, the second 712.