315/45R20 vs 295/50R20
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/45R20 | 295/50R20 | Change |
|---|---|---|---|
| Overall height | 31.2" | 31.6" | +1.5 % |
| Sidewall | 5.58" | 5.81" | +4.1 % |
| Section width | 315 mm | 295 mm | -6.3 % |
| Circumference | 97.9" | 99.3" | +1.5 % |
| Revs per mile | 647 | 638 | -1.5 % |
| Axle height | 15.58" | 15.81" | +5.8 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 315/45R20. Fit 295/50R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,145 on the road, so it under-counts by 145. At a true 70 mph the first turns 755 times a minute, the second 744.