225/70R20 vs 205/75R20
Going from one to the other changes the overall height by -0.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/70R20 | 205/75R20 | Change |
|---|---|---|---|
| Overall height | 32.4" | 32.1" | -0.9 % |
| Sidewall | 6.20" | 6.05" | -2.4 % |
| Section width | 225 mm | 205 mm | -8.9 % |
| Circumference | 101.8" | 100.9" | -0.9 % |
| Revs per mile | 622 | 628 | +0.9 % |
| Axle height | 16.20" | 16.05" | -3.8 mm |
| Approved rim width | 6–8.5" | 5.5–7.5" | overlap 6–7.5" |
Both are approved on a rim 6" to 7.5" 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 205/75R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.2 mph | -0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,909 on the road, so it over-counts by 91. At a true 70 mph the first turns 726 times a minute, the second 733.