235/45R21 vs 215/45R21
Going from one to the other changes the overall height by -2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/45R21 | 215/45R21 | Change |
|---|---|---|---|
| Overall height | 29.3" | 28.6" | -2.4 % |
| Sidewall | 4.16" | 3.81" | -8.5 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 92.1" | 89.9" | -2.4 % |
| Revs per mile | 688 | 705 | +2.4 % |
| Axle height | 14.66" | 14.31" | -9.0 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 235/45R21. Fit 215/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.8 mph | -2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,758 on the road, so it over-counts by 242. At a true 70 mph the first turns 802 times a minute, the second 822.