245/75R20 vs 225/80R20
Going from one to the other changes the overall height by -0.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/75R20 | 225/80R20 | Change |
|---|---|---|---|
| Overall height | 34.5" | 34.2" | -0.9 % |
| Sidewall | 7.23" | 7.09" | -2.0 % |
| Section width | 245 mm | 225 mm | -8.2 % |
| Circumference | 108.3" | 107.4" | -0.9 % |
| Revs per mile | 585 | 590 | +0.9 % |
| Axle height | 17.23" | 17.09" | -3.8 mm |
| Approved rim width | 7–9" | 6–8.5" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/75R20. Fit 225/80R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.2 mph | -0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,914 on the road, so it over-counts by 86. At a true 70 mph the first turns 683 times a minute, the second 689.