225/40R20 vs 215/40R20
Going from one to the other changes the overall height by -1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/40R20 | 215/40R20 | Change |
|---|---|---|---|
| Overall height | 27.1" | 26.8" | -1.2 % |
| Sidewall | 3.54" | 3.39" | -4.4 % |
| Section width | 225 mm | 215 mm | -4.4 % |
| Circumference | 85.1" | 84.1" | -1.2 % |
| Revs per mile | 745 | 753 | +1.2 % |
| Axle height | 13.54" | 13.39" | -4.0 mm |
| Approved rim width | 6–8.5" | 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 225/40R20. Fit 215/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,884 on the road, so it over-counts by 116. At a true 70 mph the first turns 869 times a minute, the second 879.