285/60R23 vs 305/60R23
Going from one to the other changes the overall height by +2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/60R23 | 305/60R23 | Change |
|---|---|---|---|
| Overall height | 36.5" | 37.4" | +2.6 % |
| Sidewall | 6.73" | 7.20" | +7.0 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 114.6" | 117.5" | +2.6 % |
| Revs per mile | 553 | 539 | -2.6 % |
| Axle height | 18.23" | 18.70" | +12.0 mm |
| Approved rim width | 8–10.5" | 8.5–11" | 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 285/60R23. Fit 305/60R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,259 on the road, so it under-counts by 259. At a true 70 mph the first turns 645 times a minute, the second 629.