305/70R22.5 vs 285/80R22.5
Going from one to the other changes the overall height by +2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/70R22.5 | 285/80R22.5 | Change |
|---|---|---|---|
| Overall height | 39.3" | 40.5" | +2.9 % |
| Sidewall | 8.41" | 8.98" | +6.8 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 123.5" | 127.1" | +2.9 % |
| Revs per mile | 513 | 499 | -2.9 % |
| Axle height | 19.66" | 20.23" | +14.5 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 305/70R22.5. Fit 285/80R22.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,290 on the road, so it under-counts by 290. At a true 70 mph the first turns 599 times a minute, the second 582.