255/55R21 vs 245/55R21
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/55R21 | 245/55R21 | Change |
|---|---|---|---|
| Overall height | 32.0" | 31.6" | -1.4 % |
| Sidewall | 5.52" | 5.31" | -3.9 % |
| Section width | 255 mm | 245 mm | -3.9 % |
| Circumference | 100.7" | 99.3" | -1.4 % |
| Revs per mile | 629 | 638 | +1.4 % |
| Axle height | 16.02" | 15.81" | -5.5 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 245/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,865 on the road, so it over-counts by 135. At a true 70 mph the first turns 734 times a minute, the second 744.