225/75R20 vs 205/80R20
Going from one to the other changes the overall height by -1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/75R20 | 205/80R20 | Change |
|---|---|---|---|
| Overall height | 33.3" | 32.9" | -1.1 % |
| Sidewall | 6.64" | 6.46" | -2.8 % |
| Section width | 225 mm | 205 mm | -8.9 % |
| Circumference | 104.6" | 103.4" | -1.1 % |
| Revs per mile | 606 | 613 | +1.1 % |
| Axle height | 16.64" | 16.46" | -4.8 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 225/75R20. Fit 205/80R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,888 on the road, so it over-counts by 112. At a true 70 mph the first turns 707 times a minute, the second 715.