225/75R20 vs 245/70R20
Going from one to the other changes the overall height by +0.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/75R20 | 245/70R20 | Change |
|---|---|---|---|
| Overall height | 33.3" | 33.5" | +0.7 % |
| Sidewall | 6.64" | 6.75" | +1.6 % |
| Section width | 225 mm | 245 mm | +8.9 % |
| Circumference | 104.6" | 105.3" | +0.7 % |
| Revs per mile | 606 | 602 | -0.7 % |
| Axle height | 16.64" | 16.75" | +2.8 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 225/75R20. Fit 245/70R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.3 mph | +0.3 |
| 70 mph | 70.5 mph | +0.5 |
| 90 mph | 90.6 mph | +0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,065 on the road, so it under-counts by 65. At a true 70 mph the first turns 707 times a minute, the second 702.