225/45R20 vs 245/40R20
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 | 225/45R20 | 245/40R20 | Change |
|---|---|---|---|
| Overall height | 28.0" | 27.7" | -0.9 % |
| Sidewall | 3.99" | 3.86" | -3.2 % |
| Section width | 225 mm | 245 mm | +8.9 % |
| Circumference | 87.9" | 87.1" | -0.9 % |
| Revs per mile | 721 | 728 | +0.9 % |
| Axle height | 13.99" | 13.86" | -3.3 mm |
| Approved rim width | 6–8.5" | 7–9" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 225/45R20. Fit 245/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.2 mph | -0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,909 on the road, so it over-counts by 91. At a true 70 mph the first turns 841 times a minute, the second 849.
What drivers reported
225/45R20 carries 1 complaints. Nobody has filed one naming 245/40R20.