305/75R22.5 vs 285/80R22.5
Going from one to the other changes the overall height by -0.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/75R22.5 | 285/80R22.5 | Change |
|---|---|---|---|
| Overall height | 40.5" | 40.5" | -0.1 % |
| Sidewall | 9.01" | 8.98" | -0.3 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 127.3" | 127.1" | -0.1 % |
| Revs per mile | 498 | 499 | +0.1 % |
| Axle height | 20.26" | 20.23" | -0.8 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 305/75R22.5. Fit 285/80R22.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | -0.0 |
| 50 mph | 49.9 mph | -0.1 |
| 70 mph | 69.9 mph | -0.1 |
| 90 mph | 89.9 mph | -0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,985 on the road, so it over-counts by 15. At a true 70 mph the first turns 581 times a minute, the second 582.