315/40R20 vs 295/45R20
Going from one to the other changes the overall height by +1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/40R20 | 295/45R20 | Change |
|---|---|---|---|
| Overall height | 29.9" | 30.5" | +1.8 % |
| Sidewall | 4.96" | 5.23" | +5.4 % |
| Section width | 315 mm | 295 mm | -6.3 % |
| Circumference | 94.0" | 95.7" | +1.8 % |
| Revs per mile | 674 | 662 | -1.8 % |
| Axle height | 14.96" | 15.23" | +6.8 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 315/40R20. Fit 295/45R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.6 mph | +1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,178 on the road, so it under-counts by 178. At a true 70 mph the first turns 786 times a minute, the second 773.