235/35R21 vs 215/35R21
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 | 235/35R21 | 215/35R21 | Change |
|---|---|---|---|
| Overall height | 27.5" | 26.9" | -2.0 % |
| Sidewall | 3.24" | 2.96" | -8.5 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 86.3" | 84.6" | -2.0 % |
| Revs per mile | 734 | 749 | +2.0 % |
| Axle height | 13.74" | 13.46" | -7.0 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 235/35R21. Fit 215/35R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,799 on the road, so it over-counts by 201. At a true 70 mph the first turns 856 times a minute, the second 874.