235/40R18 vs 245/35R18
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 | 235/40R18 | 245/35R18 | Change |
|---|---|---|---|
| Overall height | 25.4" | 24.8" | -2.6 % |
| Sidewall | 3.70" | 3.38" | -8.8 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 79.8" | 77.8" | -2.6 % |
| Revs per mile | 794 | 815 | +2.6 % |
| Axle height | 12.70" | 12.38" | -8.3 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 235/40R18. Fit 245/35R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,744 on the road, so it over-counts by 256. At a true 70 mph the first turns 926 times a minute, the second 951.
What drivers reported
235/40R18 carries 23 complaints. Nobody has filed one naming 245/35R18.