295/55R21 vs 275/55R21
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/55R21 | 275/55R21 | Change |
|---|---|---|---|
| Overall height | 33.8" | 32.9" | -2.6 % |
| Sidewall | 6.39" | 5.95" | -6.8 % |
| Section width | 295 mm | 275 mm | -6.8 % |
| Circumference | 106.1" | 103.4" | -2.6 % |
| Revs per mile | 597 | 613 | +2.6 % |
| Axle height | 16.89" | 16.45" | -11.0 mm |
| Approved rim width | 8.5–11" | 7.5–10" | overlap 8.5–10" |
Both are approved on a rim 8.5" to 10" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 295/55R21. Fit 275/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,744 on the road, so it over-counts by 256. At a true 70 mph the first turns 697 times a minute, the second 715.