295/60R18 vs 305/60R18
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/60R18 | 305/60R18 | Change |
|---|---|---|---|
| Overall height | 31.9" | 32.4" | +1.5 % |
| Sidewall | 6.97" | 7.20" | +3.4 % |
| Section width | 295 mm | 305 mm | +3.4 % |
| Circumference | 100.3" | 101.8" | +1.5 % |
| Revs per mile | 631 | 622 | -1.5 % |
| Axle height | 15.97" | 16.20" | +6.0 mm |
| Approved rim width | 8.5–11" | 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 295/60R18. Fit 305/60R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,148 on the road, so it under-counts by 148. At a true 70 mph the first turns 737 times a minute, the second 726.