275/30R18 vs 255/35R18
Going from one to the other changes the overall height by +2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/30R18 | 255/35R18 | Change |
|---|---|---|---|
| Overall height | 24.5" | 25.0" | +2.2 % |
| Sidewall | 3.25" | 3.51" | +8.2 % |
| Section width | 275 mm | 255 mm | -7.3 % |
| Circumference | 77.0" | 78.6" | +2.2 % |
| Revs per mile | 823 | 806 | -2.2 % |
| Axle height | 12.25" | 12.51" | +6.8 mm |
| Approved rim width | 7.5–10" | 7–9.5" | 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 275/30R18. Fit 255/35R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.5 mph | +1.5 |
| 90 mph | 92.0 mph | +2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,217 on the road, so it under-counts by 217. At a true 70 mph the first turns 961 times a minute, the second 940.
What drivers reported
Nobody has filed a complaint naming 275/30R18. 255/35R18 carries 12.