215/40R18 vs 225/40R18
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 | 215/40R18 | 225/40R18 | Change |
|---|---|---|---|
| Overall height | 24.8" | 25.1" | +1.3 % |
| Sidewall | 3.39" | 3.54" | +4.7 % |
| Section width | 215 mm | 225 mm | +4.7 % |
| Circumference | 77.8" | 78.8" | +1.3 % |
| Revs per mile | 814 | 804 | -1.3 % |
| Axle height | 12.39" | 12.54" | +4.0 mm |
| Approved rim width | 6–8" | 6–8.5" | overlap 6–8" |
Both are approved on a rim 6" to 8" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 215/40R18. Fit 225/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.9 mph | +0.9 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,127 on the road, so it under-counts by 127. At a true 70 mph the first turns 950 times a minute, the second 938.