235/45R18 vs 225/45R18
Going from one to the other changes the overall height by -1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/45R18 | 225/45R18 | Change |
|---|---|---|---|
| Overall height | 26.3" | 26.0" | -1.3 % |
| Sidewall | 4.16" | 3.99" | -4.3 % |
| Section width | 235 mm | 225 mm | -4.3 % |
| Circumference | 82.7" | 81.6" | -1.3 % |
| Revs per mile | 766 | 777 | +1.3 % |
| Axle height | 13.16" | 12.99" | -4.5 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 235/45R18. Fit 225/45R18 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 | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,865 on the road, so it over-counts by 135. At a true 70 mph the first turns 894 times a minute, the second 906.