245/70R20 vs 225/75R20
Going from one to the other changes the overall height by -0.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/70R20 | 225/75R20 | Change |
|---|---|---|---|
| Overall height | 33.5" | 33.3" | -0.6 % |
| Sidewall | 6.75" | 6.64" | -1.6 % |
| Section width | 245 mm | 225 mm | -8.2 % |
| Circumference | 105.3" | 104.6" | -0.6 % |
| Revs per mile | 602 | 606 | +0.6 % |
| Axle height | 16.75" | 16.64" | -2.8 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 245/70R20. Fit 225/75R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.7 mph | -0.3 |
| 70 mph | 69.5 mph | -0.5 |
| 90 mph | 89.4 mph | -0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,935 on the road, so it over-counts by 65. At a true 70 mph the first turns 702 times a minute, the second 707.