245/65R20 vs 225/75R20
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 | 245/65R20 | 225/75R20 | Change |
|---|---|---|---|
| Overall height | 32.5" | 33.3" | +2.3 % |
| Sidewall | 6.27" | 6.64" | +6.0 % |
| Section width | 245 mm | 225 mm | -8.2 % |
| Circumference | 102.2" | 104.6" | +2.3 % |
| Revs per mile | 620 | 606 | -2.3 % |
| Axle height | 16.27" | 16.64" | +9.5 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 245/65R20. Fit 225/75R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.6 mph | +1.6 |
| 90 mph | 92.1 mph | +2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,230 on the road, so it under-counts by 230. At a true 70 mph the first turns 723 times a minute, the second 707.