225/70R17.5 vs 215/75R17.5
Going from one to the other changes the overall height by +1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/70R17.5 | 215/75R17.5 | Change |
|---|---|---|---|
| Overall height | 29.9" | 30.2" | +1.0 % |
| Sidewall | 6.20" | 6.35" | +2.4 % |
| Section width | 225 mm | 215 mm | -4.4 % |
| Circumference | 93.9" | 94.9" | +1.0 % |
| Revs per mile | 674 | 668 | -1.0 % |
| Axle height | 14.95" | 15.10" | +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 225/70R17.5. Fit 215/75R17.5 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 | 70.7 mph | +0.7 |
| 90 mph | 90.9 mph | +0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,099 on the road, so it under-counts by 99. At a true 70 mph the first turns 787 times a minute, the second 779.