215/45R18 vs 235/45R18
Going from one to the other changes the overall height by +2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/45R18 | 235/45R18 | Change |
|---|---|---|---|
| Overall height | 25.6" | 26.3" | +2.8 % |
| Sidewall | 3.81" | 4.16" | +9.3 % |
| Section width | 215 mm | 235 mm | +9.3 % |
| Circumference | 80.5" | 82.7" | +2.8 % |
| Revs per mile | 787 | 766 | -2.8 % |
| Axle height | 12.81" | 13.16" | +9.0 mm |
| Approved rim width | 6–8" | 6.5–8.5" | 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 215/45R18. Fit 235/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.5 mph | +2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,277 on the road, so it under-counts by 277. At a true 70 mph the first turns 918 times a minute, the second 894.