305/50R17 vs 285/55R17
Going from one to the other changes the overall height by +1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/50R17 | 285/55R17 | Change |
|---|---|---|---|
| Overall height | 29.0" | 29.3" | +1.2 % |
| Sidewall | 6.00" | 6.17" | +2.8 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 91.1" | 92.2" | +1.2 % |
| Revs per mile | 695 | 687 | -1.2 % |
| Axle height | 14.50" | 14.67" | +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 305/50R17. Fit 285/55R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,115 on the road, so it under-counts by 115. At a true 70 mph the first turns 811 times a minute, the second 802.