255/55R21 vs 245/60R21
Going from one to the other changes the overall height by +1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/55R21 | 245/60R21 | Change |
|---|---|---|---|
| Overall height | 32.0" | 32.6" | +1.7 % |
| Sidewall | 5.52" | 5.79" | +4.8 % |
| Section width | 255 mm | 245 mm | -3.9 % |
| Circumference | 100.7" | 102.3" | +1.7 % |
| Revs per mile | 629 | 619 | -1.7 % |
| Axle height | 16.02" | 16.29" | +6.8 mm |
| Approved rim width | 7–9.5" | 7–9" | 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 255/55R21. Fit 245/60R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,166 on the road, so it under-counts by 166. At a true 70 mph the first turns 734 times a minute, the second 722.