305/50R24 vs 285/55R24
Going from one to the other changes the overall height by +0.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/50R24 | 285/55R24 | Change |
|---|---|---|---|
| Overall height | 36.0" | 36.3" | +0.9 % |
| Sidewall | 6.00" | 6.17" | +2.8 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 113.1" | 114.2" | +0.9 % |
| Revs per mile | 560 | 555 | -0.9 % |
| Axle height | 18.00" | 18.17" | +4.3 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 305/50R24. Fit 285/55R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.7 mph | +0.7 |
| 90 mph | 90.8 mph | +0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,093 on the road, so it under-counts by 93. At a true 70 mph the first turns 653 times a minute, the second 647.