315/50R18 vs 305/55R18
Going from one to the other changes the overall height by +2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/50R18 | 305/55R18 | Change |
|---|---|---|---|
| Overall height | 30.4" | 31.2" | +2.7 % |
| Sidewall | 6.20" | 6.60" | +6.5 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 95.5" | 98.0" | +2.7 % |
| Revs per mile | 663 | 646 | -2.7 % |
| Axle height | 15.20" | 15.60" | +10.3 mm |
| Approved rim width | 8.5–11.5" | 8.5–11" | overlap 8.5–11" |
Both are approved on a rim 8.5" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 315/50R18. Fit 305/55R18 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 | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,265 on the road, so it under-counts by 265. At a true 70 mph the first turns 774 times a minute, the second 754.