225/70R21 vs 235/70R21
Going from one to the other changes the overall height by +1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/70R21 | 235/70R21 | Change |
|---|---|---|---|
| Overall height | 33.4" | 34.0" | +1.7 % |
| Sidewall | 6.20" | 6.48" | +4.4 % |
| Section width | 225 mm | 235 mm | +4.4 % |
| Circumference | 104.9" | 106.7" | +1.7 % |
| Revs per mile | 604 | 594 | -1.7 % |
| Axle height | 16.70" | 16.98" | +7.0 mm |
| Approved rim width | 6–8.5" | 6.5–8.5" | 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 225/70R21. Fit 235/70R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,165 on the road, so it under-counts by 165. At a true 70 mph the first turns 704 times a minute, the second 693.