235/70R21 vs 225/75R21
Going from one to the other changes the overall height by +1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/70R21 | 225/75R21 | Change |
|---|---|---|---|
| Overall height | 34.0" | 34.3" | +1.0 % |
| Sidewall | 6.48" | 6.64" | +2.6 % |
| Section width | 235 mm | 225 mm | -4.3 % |
| Circumference | 106.7" | 107.7" | +1.0 % |
| Revs per mile | 594 | 588 | -1.0 % |
| Axle height | 16.98" | 17.14" | +4.3 mm |
| Approved rim width | 6.5–8.5" | 6–8.5" | overlap 6.5–8.5" |
Both are approved on a rim 6.5" 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 235/70R21. Fit 225/75R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.7 mph | +0.7 |
| 90 mph | 90.9 mph | +0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,099 on the road, so it under-counts by 99. At a true 70 mph the first turns 693 times a minute, the second 686.