235/50R18 vs 225/50R18
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/50R18 | 225/50R18 | Change |
|---|---|---|---|
| Overall height | 27.3" | 26.9" | -1.4 % |
| Sidewall | 4.63" | 4.43" | -4.3 % |
| Section width | 235 mm | 225 mm | -4.3 % |
| Circumference | 85.6" | 84.4" | -1.4 % |
| Revs per mile | 740 | 751 | +1.4 % |
| Axle height | 13.63" | 13.43" | -5.0 mm |
| Approved rim width | 6.5–8.5" | 6–8.5" | overlap 6.5–8.5" |
Both are approved on a rim 6.5" 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 235/50R18. Fit 225/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,856 on the road, so it over-counts by 144. At a true 70 mph the first turns 863 times a minute, the second 876.