235/40R18 vs 215/40R18
Going from one to the other changes the overall height by -2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/40R18 | 215/40R18 | Change |
|---|---|---|---|
| Overall height | 25.4" | 24.8" | -2.5 % |
| Sidewall | 3.70" | 3.39" | -8.5 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 79.8" | 77.8" | -2.5 % |
| Revs per mile | 794 | 814 | +2.5 % |
| Axle height | 12.70" | 12.39" | -8.0 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 235/40R18. Fit 215/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.8 mph | -2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,752 on the road, so it over-counts by 248. At a true 70 mph the first turns 926 times a minute, the second 950.