225/85R17 vs 245/75R17
Going from one to the other changes the overall height by -1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/85R17 | 245/75R17 | Change |
|---|---|---|---|
| Overall height | 32.1" | 31.5" | -1.8 % |
| Sidewall | 7.53" | 7.23" | -3.9 % |
| Section width | 225 mm | 245 mm | +8.9 % |
| Circumference | 100.7" | 98.9" | -1.8 % |
| Revs per mile | 629 | 641 | +1.8 % |
| Axle height | 16.03" | 15.73" | -7.5 mm |
| Approved rim width | 6–8.5" | 7–9" | overlap 7–8.5" |
Both are approved on a rim 7" 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 225/85R17. Fit 245/75R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,816 on the road, so it over-counts by 184. At a true 70 mph the first turns 734 times a minute, the second 748.