275/30R24 vs 285/30R24
Going from one to the other changes the overall height by +0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/30R24 | 285/30R24 | Change |
|---|---|---|---|
| Overall height | 30.5" | 30.7" | +0.8 % |
| Sidewall | 3.25" | 3.37" | +3.6 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 95.8" | 96.5" | +0.8 % |
| Revs per mile | 661 | 656 | -0.8 % |
| Axle height | 15.25" | 15.37" | +3.0 mm |
| Approved rim width | 7.5–10" | 8–10.5" | overlap 8–10" |
Both are approved on a rim 8" to 10" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 275/30R24. Fit 285/30R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.5 mph | +0.5 |
| 90 mph | 90.7 mph | +0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,077 on the road, so it under-counts by 77. At a true 70 mph the first turns 772 times a minute, the second 766.