235/35R18 vs 245/35R18
Going from one to the other changes the overall height by +1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/35R18 | 245/35R18 | Change |
|---|---|---|---|
| Overall height | 24.5" | 24.8" | +1.1 % |
| Sidewall | 3.24" | 3.38" | +4.3 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 76.9" | 77.8" | +1.1 % |
| Revs per mile | 824 | 815 | -1.1 % |
| Axle height | 12.24" | 12.38" | +3.5 mm |
| Approved rim width | 6.5–8.5" | 7–9" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/35R18. Fit 245/35R18 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 10,113 on the road, so it under-counts by 113. At a true 70 mph the first turns 961 times a minute, the second 951.