255/40R15 vs 275/40R15
Going from one to the other changes the overall height by +2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/40R15 | 275/40R15 | Change |
|---|---|---|---|
| Overall height | 23.0" | 23.7" | +2.7 % |
| Sidewall | 4.02" | 4.33" | +7.8 % |
| Section width | 255 mm | 275 mm | +7.8 % |
| Circumference | 72.4" | 74.3" | +2.7 % |
| Revs per mile | 876 | 852 | -2.7 % |
| Axle height | 11.52" | 11.83" | +8.0 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 255/40R15. Fit 275/40R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.5 mph | +2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,274 on the road, so it under-counts by 274. At a true 70 mph the first turns 1022 times a minute, the second 994.