215/75R17.5 vs 215/70R17.5
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/75R17.5 | 215/70R17.5 | Change |
|---|---|---|---|
| Overall height | 30.2" | 29.4" | -2.8 % |
| Sidewall | 6.35" | 5.93" | -6.7 % |
| Section width | 215 mm | 215 mm | 0.0 % |
| Circumference | 94.9" | 92.2" | -2.8 % |
| Revs per mile | 668 | 687 | +2.8 % |
| Axle height | 15.10" | 14.68" | -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 215/75R17.5. Fit 215/70R17.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,720 on the road, so it over-counts by 280. At a true 70 mph the first turns 779 times a minute, the second 802.