295/45R21 vs 275/50R21
Going from one to the other changes the overall height by +1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/45R21 | 275/50R21 | Change |
|---|---|---|---|
| Overall height | 31.5" | 31.8" | +1.2 % |
| Sidewall | 5.23" | 5.41" | +3.6 % |
| Section width | 295 mm | 275 mm | -6.8 % |
| Circumference | 98.8" | 100.0" | +1.2 % |
| Revs per mile | 641 | 634 | -1.2 % |
| Axle height | 15.73" | 15.91" | +4.8 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 295/45R21. Fit 275/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,119 on the road, so it under-counts by 119. At a true 70 mph the first turns 748 times a minute, the second 739.