215/45R21 vs 215/40R21
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/45R21 | 215/40R21 | Change |
|---|---|---|---|
| Overall height | 28.6" | 27.8" | -3.0 % |
| Sidewall | 3.81" | 3.39" | -11.1 % |
| Section width | 215 mm | 215 mm | 0.0 % |
| Circumference | 89.9" | 87.2" | -3.0 % |
| Revs per mile | 705 | 726 | +3.0 % |
| Axle height | 14.31" | 13.89" | -10.8 mm |
| Approved rim width | 6–8" | 6–8" | overlap 6–8" |
Both are approved on a rim 6" to 8" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 215/45R21. Fit 215/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,704 on the road, so it over-counts by 296. At a true 70 mph the first turns 822 times a minute, the second 847.