225/40R18 vs 205/40R18
Going from one to the other changes the overall height by -2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/40R18 | 205/40R18 | Change |
|---|---|---|---|
| Overall height | 25.1" | 24.5" | -2.5 % |
| Sidewall | 3.54" | 3.23" | -8.9 % |
| Section width | 225 mm | 205 mm | -8.9 % |
| Circumference | 78.8" | 76.8" | -2.5 % |
| Revs per mile | 804 | 825 | +2.5 % |
| Axle height | 12.54" | 12.23" | -8.0 mm |
| Approved rim width | 6–8.5" | 5.5–7.5" | overlap 6–7.5" |
Both are approved on a rim 6" to 7.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 225/40R18. Fit 205/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,749 on the road, so it over-counts by 251. At a true 70 mph the first turns 938 times a minute, the second 962.
What drivers reported
225/40R18 carries 39 complaints. Nobody has filed one naming 205/40R18.