305/35R20 vs 285/40R20
Going from one to the other changes the overall height by +2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/35R20 | 285/40R20 | Change |
|---|---|---|---|
| Overall height | 28.4" | 29.0" | +2.0 % |
| Sidewall | 4.20" | 4.49" | +6.8 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 89.2" | 91.0" | +2.0 % |
| Revs per mile | 710 | 696 | -2.0 % |
| Axle height | 14.20" | 14.49" | +7.3 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 305/35R20. Fit 285/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,201 on the road, so it under-counts by 201. At a true 70 mph the first turns 828 times a minute, the second 812.