275/40R21 vs 255/40R21
Going from one to the other changes the overall height by -2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/40R21 | 255/40R21 | Change |
|---|---|---|---|
| Overall height | 29.7" | 29.0" | -2.1 % |
| Sidewall | 4.33" | 4.02" | -7.3 % |
| Section width | 275 mm | 255 mm | -7.3 % |
| Circumference | 93.2" | 91.2" | -2.1 % |
| Revs per mile | 680 | 695 | +2.1 % |
| Axle height | 14.83" | 14.52" | -8.0 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 275/40R21. Fit 255/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,788 on the road, so it over-counts by 212. At a true 70 mph the first turns 793 times a minute, the second 810.