235/65R21 vs 215/75R21
Going from one to the other changes the overall height by +2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/65R21 | 215/75R21 | Change |
|---|---|---|---|
| Overall height | 33.0" | 33.7" | +2.0 % |
| Sidewall | 6.01" | 6.35" | +5.6 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 103.8" | 105.9" | +2.0 % |
| Revs per mile | 611 | 599 | -2.0 % |
| Axle height | 16.51" | 16.85" | +8.5 mm |
| Approved rim width | 6.5–8.5" | 6–8" | overlap 6.5–8" |
Both are approved on a rim 6.5" to 8" 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 215/75R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,203 on the road, so it under-counts by 203. At a true 70 mph the first turns 712 times a minute, the second 698.