245/85R18 vs 235/85R18
Going from one to the other changes the overall height by -1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/85R18 | 235/85R18 | Change |
|---|---|---|---|
| Overall height | 34.4" | 33.7" | -1.9 % |
| Sidewall | 8.20" | 7.86" | -4.1 % |
| Section width | 245 mm | 235 mm | -4.1 % |
| Circumference | 108.1" | 106.0" | -1.9 % |
| Revs per mile | 586 | 598 | +1.9 % |
| Axle height | 17.20" | 16.86" | -8.5 mm |
| Approved rim width | 7–9" | 6.5–8.5" | 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 245/85R18. Fit 235/85R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,805 on the road, so it over-counts by 195. At a true 70 mph the first turns 684 times a minute, the second 698.
What drivers reported
245/85R18 carries 2 complaints. Nobody has filed one naming 235/85R18.