305/65R20 vs 285/70R20
Going from one to the other changes the overall height by +0.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/65R20 | 285/70R20 | Change |
|---|---|---|---|
| Overall height | 35.6" | 35.7" | +0.3 % |
| Sidewall | 7.81" | 7.85" | +0.6 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 111.9" | 112.2" | +0.3 % |
| Revs per mile | 566 | 565 | -0.3 % |
| Axle height | 17.81" | 17.85" | +1.3 mm |
| Approved rim width | 8.5–11" | 8–10.5" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/65R20. Fit 285/70R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.1 mph | +0.1 |
| 70 mph | 70.2 mph | +0.2 |
| 90 mph | 90.2 mph | +0.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,028 on the road, so it under-counts by 28. At a true 70 mph the first turns 661 times a minute, the second 659.