245/60R20 vs 225/70R20
Going from one to the other changes the overall height by +2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/60R20 | 225/70R20 | Change |
|---|---|---|---|
| Overall height | 31.6" | 32.4" | +2.6 % |
| Sidewall | 5.79" | 6.20" | +7.1 % |
| Section width | 245 mm | 225 mm | -8.2 % |
| Circumference | 99.2" | 101.8" | +2.6 % |
| Revs per mile | 639 | 622 | -2.6 % |
| Axle height | 15.79" | 16.20" | +10.5 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 245/60R20. Fit 225/70R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,262 on the road, so it under-counts by 262. At a true 70 mph the first turns 745 times a minute, the second 726.