225/70R20 vs 215/75R20
Going from one to the other changes the overall height by +0.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/70R20 | 215/75R20 | Change |
|---|---|---|---|
| Overall height | 32.4" | 32.7" | +0.9 % |
| Sidewall | 6.20" | 6.35" | +2.4 % |
| Section width | 225 mm | 215 mm | -4.4 % |
| Circumference | 101.8" | 102.7" | +0.9 % |
| Revs per mile | 622 | 617 | -0.9 % |
| Axle height | 16.20" | 16.35" | +3.8 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 225/70R20. Fit 215/75R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.6 mph | +0.6 |
| 90 mph | 90.8 mph | +0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,091 on the road, so it under-counts by 91. At a true 70 mph the first turns 726 times a minute, the second 720.