255/65R21 vs 245/70R21
Going from one to the other changes the overall height by +1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/65R21 | 245/70R21 | Change |
|---|---|---|---|
| Overall height | 34.1" | 34.5" | +1.3 % |
| Sidewall | 6.53" | 6.75" | +3.5 % |
| Section width | 255 mm | 245 mm | -3.9 % |
| Circumference | 107.0" | 108.4" | +1.3 % |
| Revs per mile | 592 | 585 | -1.3 % |
| Axle height | 17.03" | 17.25" | +5.8 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 255/65R21. Fit 245/70R21 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 | 70.9 mph | +0.9 |
| 90 mph | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,133 on the road, so it under-counts by 133. At a true 70 mph the first turns 691 times a minute, the second 682.