315/55R20 vs 295/60R20
Going from one to the other changes the overall height by +0.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/55R20 | 295/60R20 | Change |
|---|---|---|---|
| Overall height | 33.6" | 33.9" | +0.9 % |
| Sidewall | 6.82" | 6.97" | +2.2 % |
| Section width | 315 mm | 295 mm | -6.3 % |
| Circumference | 105.7" | 106.6" | +0.9 % |
| Revs per mile | 599 | 594 | -0.9 % |
| Axle height | 16.82" | 16.97" | +3.8 mm |
| Approved rim width | 8.5–11.5" | 8.5–11" | overlap 8.5–11" |
Both are approved on a rim 8.5" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 315/55R20. Fit 295/60R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.6 mph | +0.6 |
| 90 mph | 90.8 mph | +0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,088 on the road, so it under-counts by 88. At a true 70 mph the first turns 699 times a minute, the second 693.