245/35R18 vs 255/30R18
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/35R18 | 255/30R18 | Change |
|---|---|---|---|
| Overall height | 24.8" | 24.0" | -2.9 % |
| Sidewall | 3.38" | 3.01" | -10.8 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 77.8" | 75.5" | -2.9 % |
| Revs per mile | 815 | 840 | +2.9 % |
| Axle height | 12.38" | 12.01" | -9.3 mm |
| Approved rim width | 7–9" | 7–9.5" | overlap 7–9" |
Both are approved on a rim 7" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/35R18. Fit 255/30R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,706 on the road, so it over-counts by 294. At a true 70 mph the first turns 951 times a minute, the second 979.