235/65R21 vs 225/70R21
Going from one to the other changes the overall height by +1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/65R21 | 225/70R21 | Change |
|---|---|---|---|
| Overall height | 33.0" | 33.4" | +1.1 % |
| Sidewall | 6.01" | 6.20" | +3.1 % |
| Section width | 235 mm | 225 mm | -4.3 % |
| Circumference | 103.8" | 104.9" | +1.1 % |
| Revs per mile | 611 | 604 | -1.1 % |
| Axle height | 16.51" | 16.70" | +4.8 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 235/65R21. Fit 225/70R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,113 on the road, so it under-counts by 113. At a true 70 mph the first turns 712 times a minute, the second 704.