235/55R21 vs 215/60R21
Going from one to the other changes the overall height by -0.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/55R21 | 215/60R21 | Change |
|---|---|---|---|
| Overall height | 31.2" | 31.2" | -0.1 % |
| Sidewall | 5.09" | 5.08" | -0.2 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 97.9" | 97.9" | -0.1 % |
| Revs per mile | 647 | 647 | +0.1 % |
| Axle height | 15.59" | 15.58" | -0.3 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 235/55R21. Fit 215/60R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | -0.0 |
| 50 mph | 50.0 mph | -0.0 |
| 70 mph | 70.0 mph | -0.0 |
| 90 mph | 89.9 mph | -0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,994 on the road, so it over-counts by 6. At a true 70 mph the first turns 755 times a minute, the second 755.