235/60R21 vs 215/70R21
Going from one to the other changes the overall height by +2.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/60R21 | 215/70R21 | Change |
|---|---|---|---|
| Overall height | 32.1" | 32.9" | +2.3 % |
| Sidewall | 5.55" | 5.93" | +6.7 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 100.9" | 103.2" | +2.3 % |
| Revs per mile | 628 | 614 | -2.3 % |
| Axle height | 16.05" | 16.43" | +9.5 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 235/60R21. Fit 215/70R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.6 mph | +1.6 |
| 90 mph | 92.1 mph | +2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,233 on the road, so it under-counts by 233. At a true 70 mph the first turns 733 times a minute, the second 716.