235/45R18 vs 215/50R18
Going from one to the other changes the overall height by +0.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/45R18 | 215/50R18 | Change |
|---|---|---|---|
| Overall height | 26.3" | 26.5" | +0.5 % |
| Sidewall | 4.16" | 4.23" | +1.7 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 82.7" | 83.1" | +0.5 % |
| Revs per mile | 766 | 762 | -0.5 % |
| Axle height | 13.16" | 13.23" | +1.8 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 235/45R18. Fit 215/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.3 mph | +0.3 |
| 70 mph | 70.4 mph | +0.4 |
| 90 mph | 90.5 mph | +0.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,052 on the road, so it under-counts by 52. At a true 70 mph the first turns 894 times a minute, the second 889.
What drivers reported
235/45R18 carries 12 complaints. Nobody has filed one naming 215/50R18.