305/35R16 vs 285/40R16
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 | 305/35R16 | 285/40R16 | Change |
|---|---|---|---|
| Overall height | 24.4" | 25.0" | +2.3 % |
| Sidewall | 4.20" | 4.49" | +6.8 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 76.7" | 78.5" | +2.3 % |
| Revs per mile | 826 | 807 | -2.3 % |
| Axle height | 12.20" | 12.49" | +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 305/35R16. Fit 285/40R16 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 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 10,234 on the road, so it under-counts by 234. At a true 70 mph the first turns 964 times a minute, the second 942.