215/45R20 vs 235/40R20
Going from one to the other changes the overall height by -0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/45R20 | 235/40R20 | Change |
|---|---|---|---|
| Overall height | 27.6" | 27.4" | -0.8 % |
| Sidewall | 3.81" | 3.70" | -2.8 % |
| Section width | 215 mm | 235 mm | +9.3 % |
| Circumference | 86.8" | 86.1" | -0.8 % |
| Revs per mile | 730 | 736 | +0.8 % |
| Axle height | 13.81" | 13.70" | -2.8 mm |
| Approved rim width | 6–8" | 6.5–8.5" | overlap 6.5–8" |
Both are approved on a rim 6.5" to 8" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 215/45R20. Fit 235/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.5 mph | -0.5 |
| 90 mph | 89.3 mph | -0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,922 on the road, so it over-counts by 78. At a true 70 mph the first turns 852 times a minute, the second 859.