295/60R21 vs 275/65R21
Going from one to the other changes the overall height by +0.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/60R21 | 275/65R21 | Change |
|---|---|---|---|
| Overall height | 34.9" | 35.1" | +0.4 % |
| Sidewall | 6.97" | 7.04" | +1.0 % |
| Section width | 295 mm | 275 mm | -6.8 % |
| Circumference | 109.8" | 110.2" | +0.4 % |
| Revs per mile | 577 | 575 | -0.4 % |
| Axle height | 17.47" | 17.54" | +1.8 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 295/60R21. Fit 275/65R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.2 mph | +0.2 |
| 70 mph | 70.3 mph | +0.3 |
| 90 mph | 90.4 mph | +0.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,039 on the road, so it under-counts by 39. At a true 70 mph the first turns 673 times a minute, the second 671.