205/80R18 vs 225/75R18
Going from one to the other changes the overall height by +1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/80R18 | 225/75R18 | Change |
|---|---|---|---|
| Overall height | 30.9" | 31.3" | +1.2 % |
| Sidewall | 6.46" | 6.64" | +2.9 % |
| Section width | 205 mm | 225 mm | +9.8 % |
| Circumference | 97.1" | 98.3" | +1.2 % |
| Revs per mile | 652 | 645 | -1.2 % |
| Axle height | 15.46" | 15.64" | +4.8 mm |
| Approved rim width | 5.5–7.5" | 6–8.5" | 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 205/80R18. Fit 225/75R18 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 | 70.8 mph | +0.8 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,121 on the road, so it under-counts by 121. At a true 70 mph the first turns 761 times a minute, the second 752.