235/45R20 vs 245/40R20
Going from one to the other changes the overall height by -2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/45R20 | 245/40R20 | Change |
|---|---|---|---|
| Overall height | 28.3" | 27.7" | -2.2 % |
| Sidewall | 4.16" | 3.86" | -7.3 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 89.0" | 87.1" | -2.2 % |
| Revs per mile | 712 | 728 | +2.2 % |
| Axle height | 14.16" | 13.86" | -7.8 mm |
| Approved rim width | 6.5–8.5" | 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 235/45R20. Fit 245/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,785 on the road, so it over-counts by 215. At a true 70 mph the first turns 831 times a minute, the second 849.
What drivers reported
235/45R20 carries 2 complaints. Nobody has filed one naming 245/40R20.