235/35R18 vs 215/40R18
Going from one to the other changes the overall height by +1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/35R18 | 215/40R18 | Change |
|---|---|---|---|
| Overall height | 24.5" | 24.8" | +1.2 % |
| Sidewall | 3.24" | 3.39" | +4.6 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 76.9" | 77.8" | +1.2 % |
| Revs per mile | 824 | 814 | -1.2 % |
| Axle height | 12.24" | 12.39" | +3.8 mm |
| Approved rim width | 6.5–8.5" | 6–8" | overlap 6.5–8" |
Both are approved on a rim 6.5" to 8" 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 215/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,121 on the road, so it under-counts by 121. At a true 70 mph the first turns 961 times a minute, the second 950.
What drivers reported
Nobody has filed a complaint naming 235/35R18. 215/40R18 carries 2.