225/75R21 vs 245/70R21
Going from one to the other changes the overall height by +0.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/75R21 | 245/70R21 | Change |
|---|---|---|---|
| Overall height | 34.3" | 34.5" | +0.6 % |
| Sidewall | 6.64" | 6.75" | +1.6 % |
| Section width | 225 mm | 245 mm | +8.9 % |
| Circumference | 107.7" | 108.4" | +0.6 % |
| Revs per mile | 588 | 585 | -0.6 % |
| Axle height | 17.14" | 17.25" | +2.8 mm |
| Approved rim width | 6–8.5" | 7–9" | 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 225/75R21. Fit 245/70R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.3 mph | +0.3 |
| 70 mph | 70.4 mph | +0.4 |
| 90 mph | 90.6 mph | +0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,063 on the road, so it under-counts by 63. At a true 70 mph the first turns 686 times a minute, the second 682.