255/45R21 vs 275/45R21
Going from one to the other changes the overall height by +2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/45R21 | 275/45R21 | Change |
|---|---|---|---|
| Overall height | 30.0" | 30.7" | +2.4 % |
| Sidewall | 4.52" | 4.87" | +7.8 % |
| Section width | 255 mm | 275 mm | +7.8 % |
| Circumference | 94.4" | 96.6" | +2.4 % |
| Revs per mile | 671 | 656 | -2.4 % |
| Axle height | 15.02" | 15.37" | +9.0 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 255/45R21. Fit 275/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.1 mph | +2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,236 on the road, so it under-counts by 236. At a true 70 mph the first turns 783 times a minute, the second 765.