225/70R20 vs 245/60R20
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 | 225/70R20 | 245/60R20 | Change |
|---|---|---|---|
| Overall height | 32.4" | 31.6" | -2.6 % |
| Sidewall | 6.20" | 5.79" | -6.7 % |
| Section width | 225 mm | 245 mm | +8.9 % |
| Circumference | 101.8" | 99.2" | -2.6 % |
| Revs per mile | 622 | 639 | +2.6 % |
| Axle height | 16.20" | 15.79" | -10.5 mm |
| Approved rim width | 6–8.5" | 7–9" | 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 225/70R20. Fit 245/60R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,745 on the road, so it over-counts by 255. At a true 70 mph the first turns 726 times a minute, the second 745.