225/70R21 vs 225/75R21
Going from one to the other changes the overall height by +2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/70R21 | 225/75R21 | Change |
|---|---|---|---|
| Overall height | 33.4" | 34.3" | +2.7 % |
| Sidewall | 6.20" | 6.64" | +7.1 % |
| Section width | 225 mm | 225 mm | 0.0 % |
| Circumference | 104.9" | 107.7" | +2.7 % |
| Revs per mile | 604 | 588 | -2.7 % |
| Axle height | 16.70" | 17.14" | +11.3 mm |
| Approved rim width | 6–8.5" | 6–8.5" | overlap 6–8.5" |
Both are approved on a rim 6" 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 225/70R21. Fit 225/75R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,265 on the road, so it under-counts by 265. At a true 70 mph the first turns 704 times a minute, the second 686.