215/70R20 vs 215/75R20
Going from one to the other changes the overall height by +2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/70R20 | 215/75R20 | Change |
|---|---|---|---|
| Overall height | 31.9" | 32.7" | +2.7 % |
| Sidewall | 5.93" | 6.35" | +7.1 % |
| Section width | 215 mm | 215 mm | 0.0 % |
| Circumference | 100.1" | 102.7" | +2.7 % |
| Revs per mile | 633 | 617 | -2.7 % |
| Axle height | 15.93" | 16.35" | +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 215/70R20. Fit 215/75R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,266 on the road, so it under-counts by 266. At a true 70 mph the first turns 739 times a minute, the second 720.