215/40R20 vs 205/45R20
Going from one to the other changes the overall height by +1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/40R20 | 205/45R20 | Change |
|---|---|---|---|
| Overall height | 26.8" | 27.3" | +1.8 % |
| Sidewall | 3.39" | 3.63" | +7.3 % |
| Section width | 215 mm | 205 mm | -4.7 % |
| Circumference | 84.1" | 85.7" | +1.8 % |
| Revs per mile | 753 | 740 | -1.8 % |
| Axle height | 13.39" | 13.63" | +6.3 mm |
| Approved rim width | 6–8" | 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 215/40R20. Fit 205/45R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.3 mph | +1.3 |
| 90 mph | 91.7 mph | +1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,184 on the road, so it under-counts by 184. At a true 70 mph the first turns 879 times a minute, the second 863.