225/70R17.5 vs 225/75R17.5
Going from one to the other changes the overall height by +3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/70R17.5 | 225/75R17.5 | Change |
|---|---|---|---|
| Overall height | 29.9" | 30.8" | +3.0 % |
| Sidewall | 6.20" | 6.64" | +7.1 % |
| Section width | 225 mm | 225 mm | 0.0 % |
| Circumference | 93.9" | 96.7" | +3.0 % |
| Revs per mile | 674 | 655 | -3.0 % |
| Axle height | 14.95" | 15.39" | +11.3 mm |
| Approved rim width | 6–8.5" | 6–8.5" | overlap 6–8.5" |
Both are approved on a rim 6" to 8.5" 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 225/75R17.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.1 mph | +2.1 |
| 90 mph | 92.7 mph | +2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,296 on the road, so it under-counts by 296. At a true 70 mph the first turns 787 times a minute, the second 764.