255/75R17 vs 275/70R17
Going from one to the other changes the overall height by +0.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/75R17 | 275/70R17 | Change |
|---|---|---|---|
| Overall height | 32.1" | 32.2" | +0.3 % |
| Sidewall | 7.53" | 7.58" | +0.7 % |
| Section width | 255 mm | 275 mm | +7.8 % |
| Circumference | 100.7" | 101.0" | +0.3 % |
| Revs per mile | 629 | 627 | -0.3 % |
| Axle height | 16.03" | 16.08" | +1.3 mm |
| Approved rim width | 7–9.5" | 7.5–10" | overlap 7.5–9.5" |
Both are approved on a rim 7.5" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 255/75R17. Fit 275/70R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.2 mph | +0.2 |
| 70 mph | 70.2 mph | +0.2 |
| 90 mph | 90.3 mph | +0.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,031 on the road, so it under-counts by 31. At a true 70 mph the first turns 734 times a minute, the second 732.