215/75R21 vs 215/70R21
Going from one to the other changes the overall height by -2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/75R21 | 215/70R21 | Change |
|---|---|---|---|
| Overall height | 33.7" | 32.9" | -2.5 % |
| Sidewall | 6.35" | 5.93" | -6.7 % |
| Section width | 215 mm | 215 mm | 0.0 % |
| Circumference | 105.9" | 103.2" | -2.5 % |
| Revs per mile | 599 | 614 | +2.5 % |
| Axle height | 16.85" | 16.43" | -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 215/75R21. Fit 215/70R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,749 on the road, so it over-counts by 251. At a true 70 mph the first turns 698 times a minute, the second 716.