225/75R21 vs 225/70R21
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/75R21 | 225/70R21 | Change |
|---|---|---|---|
| Overall height | 34.3" | 33.4" | -2.6 % |
| Sidewall | 6.64" | 6.20" | -6.7 % |
| Section width | 225 mm | 225 mm | 0.0 % |
| Circumference | 107.7" | 104.9" | -2.6 % |
| Revs per mile | 588 | 604 | +2.6 % |
| Axle height | 17.14" | 16.70" | -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 225/75R21. Fit 225/70R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,742 on the road, so it over-counts by 258. At a true 70 mph the first turns 686 times a minute, the second 704.