245/40R20 vs 225/40R20
Going from one to the other changes the overall height by -2.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/40R20 | 225/40R20 | Change |
|---|---|---|---|
| Overall height | 27.7" | 27.1" | -2.3 % |
| Sidewall | 3.86" | 3.54" | -8.2 % |
| Section width | 245 mm | 225 mm | -8.2 % |
| Circumference | 87.1" | 85.1" | -2.3 % |
| Revs per mile | 728 | 745 | +2.3 % |
| Axle height | 13.86" | 13.54" | -8.0 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 245/40R20. Fit 225/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.4 mph | -1.6 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,773 on the road, so it over-counts by 227. At a true 70 mph the first turns 849 times a minute, the second 869.