225/85R21 vs 245/75R21
Going from one to the other changes the overall height by -1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/85R21 | 245/75R21 | Change |
|---|---|---|---|
| Overall height | 36.1" | 35.5" | -1.6 % |
| Sidewall | 7.53" | 7.23" | -3.9 % |
| Section width | 225 mm | 245 mm | +8.9 % |
| Circumference | 113.3" | 111.4" | -1.6 % |
| Revs per mile | 559 | 569 | +1.6 % |
| Axle height | 18.03" | 17.73" | -7.5 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 225/85R21. Fit 245/75R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.9 mph | -1.1 |
| 90 mph | 88.5 mph | -1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,836 on the road, so it over-counts by 164. At a true 70 mph the first turns 653 times a minute, the second 663.