285/45R16 vs 305/40R16
Going from one to the other changes the overall height by -1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/45R16 | 305/40R16 | Change |
|---|---|---|---|
| Overall height | 26.1" | 25.6" | -1.9 % |
| Sidewall | 5.05" | 4.80" | -4.9 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 82.0" | 80.4" | -1.9 % |
| Revs per mile | 773 | 788 | +1.9 % |
| Axle height | 13.05" | 12.80" | -6.3 mm |
| Approved rim width | 8–10.5" | 8.5–11" | 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 285/45R16. Fit 305/40R16 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,811 on the road, so it over-counts by 189. At a true 70 mph the first turns 902 times a minute, the second 919.