215/75R20 vs 215/70R20
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/75R20 | 215/70R20 | Change |
|---|---|---|---|
| Overall height | 32.7" | 31.9" | -2.6 % |
| Sidewall | 6.35" | 5.93" | -6.7 % |
| Section width | 215 mm | 215 mm | 0.0 % |
| Circumference | 102.7" | 100.1" | -2.6 % |
| Revs per mile | 617 | 633 | +2.6 % |
| Axle height | 16.35" | 15.93" | -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 215/75R20. Fit 215/70R20 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 9,741 on the road, so it over-counts by 259. At a true 70 mph the first turns 720 times a minute, the second 739.