225/40R20 vs 205/45R20
Going from one to the other changes the overall height by +0.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/40R20 | 205/45R20 | Change |
|---|---|---|---|
| Overall height | 27.1" | 27.3" | +0.7 % |
| Sidewall | 3.54" | 3.63" | +2.5 % |
| Section width | 225 mm | 205 mm | -8.9 % |
| Circumference | 85.1" | 85.7" | +0.7 % |
| Revs per mile | 745 | 740 | -0.7 % |
| Axle height | 13.54" | 13.63" | +2.3 mm |
| Approved rim width | 6–8.5" | 5.5–7.5" | overlap 6–7.5" |
Both are approved on a rim 6" 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 225/40R20. Fit 205/45R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.3 mph | +0.3 |
| 70 mph | 70.5 mph | +0.5 |
| 90 mph | 90.6 mph | +0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,065 on the road, so it under-counts by 65. At a true 70 mph the first turns 869 times a minute, the second 863.