205/45R20 vs 205/40R20
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/45R20 | 205/40R20 | Change |
|---|---|---|---|
| Overall height | 27.3" | 26.5" | -3.0 % |
| Sidewall | 3.63" | 3.23" | -11.1 % |
| Section width | 205 mm | 205 mm | 0.0 % |
| Circumference | 85.7" | 83.1" | -3.0 % |
| Revs per mile | 740 | 762 | +3.0 % |
| Axle height | 13.63" | 13.23" | -10.3 mm |
| Approved rim width | 5.5–7.5" | 5.5–7.5" | overlap 5.5–7.5" |
Both are approved on a rim 5.5" to 7.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 205/45R20. Fit 205/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,704 on the road, so it over-counts by 296. At a true 70 mph the first turns 863 times a minute, the second 889.