225/70R21 vs 215/70R21
Going from one to the other changes the overall height by -1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/70R21 | 215/70R21 | Change |
|---|---|---|---|
| Overall height | 33.4" | 32.9" | -1.7 % |
| Sidewall | 6.20" | 5.93" | -4.4 % |
| Section width | 225 mm | 215 mm | -4.4 % |
| Circumference | 104.9" | 103.2" | -1.7 % |
| Revs per mile | 604 | 614 | +1.7 % |
| Axle height | 16.70" | 16.43" | -7.0 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 225/70R21. Fit 215/70R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.8 mph | -1.2 |
| 90 mph | 88.5 mph | -1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,835 on the road, so it over-counts by 165. At a true 70 mph the first turns 704 times a minute, the second 716.