225/75R18 vs 205/85R18
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 | 225/75R18 | 205/85R18 | Change |
|---|---|---|---|
| Overall height | 31.3" | 31.7" | +1.4 % |
| Sidewall | 6.64" | 6.86" | +3.3 % |
| Section width | 225 mm | 205 mm | -8.9 % |
| Circumference | 98.3" | 99.7" | +1.4 % |
| Revs per mile | 645 | 636 | -1.4 % |
| Axle height | 15.64" | 15.86" | +5.5 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 225/75R18. Fit 205/85R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,138 on the road, so it under-counts by 138. At a true 70 mph the first turns 752 times a minute, the second 742.