245/75R20 vs 225/85R20
Going from one to the other changes the overall height by +1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/75R20 | 225/85R20 | Change |
|---|---|---|---|
| Overall height | 34.5" | 35.1" | +1.7 % |
| Sidewall | 7.23" | 7.53" | +4.1 % |
| Section width | 245 mm | 225 mm | -8.2 % |
| Circumference | 108.3" | 110.1" | +1.7 % |
| Revs per mile | 585 | 575 | -1.7 % |
| Axle height | 17.23" | 17.53" | +7.5 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 225/85R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,171 on the road, so it under-counts by 171. At a true 70 mph the first turns 683 times a minute, the second 671.