255/75R18 vs 275/65R18
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 | 255/75R18 | 275/65R18 | Change |
|---|---|---|---|
| Overall height | 33.1" | 32.1" | -3.0 % |
| Sidewall | 7.53" | 7.04" | -6.5 % |
| Section width | 255 mm | 275 mm | +7.8 % |
| Circumference | 103.9" | 100.8" | -3.0 % |
| Revs per mile | 610 | 629 | +3.0 % |
| Axle height | 16.53" | 16.04" | -12.5 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 255/75R18. Fit 275/65R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,702 on the road, so it over-counts by 298. At a true 70 mph the first turns 712 times a minute, the second 734.
What drivers reported
Nobody has filed a complaint naming 255/75R18. 275/65R18 carries 30.