245/60R20 vs 225/60R20
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/60R20 | 225/60R20 | Change |
|---|---|---|---|
| Overall height | 31.6" | 30.6" | -3.0 % |
| Sidewall | 5.79" | 5.31" | -8.2 % |
| Section width | 245 mm | 225 mm | -8.2 % |
| Circumference | 99.2" | 96.2" | -3.0 % |
| Revs per mile | 639 | 658 | +3.0 % |
| Axle height | 15.79" | 15.31" | -12.0 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 225/60R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,701 on the road, so it over-counts by 299. At a true 70 mph the first turns 745 times a minute, the second 768.