215/85R18 vs 205/85R18
Going from one to the other changes the overall height by -2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/85R18 | 205/85R18 | Change |
|---|---|---|---|
| Overall height | 32.4" | 31.7" | -2.1 % |
| Sidewall | 7.19" | 6.86" | -4.7 % |
| Section width | 215 mm | 205 mm | -4.7 % |
| Circumference | 101.8" | 99.7" | -2.1 % |
| Revs per mile | 623 | 636 | +2.1 % |
| Axle height | 16.19" | 15.86" | -8.5 mm |
| Approved rim width | 6–8" | 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 215/85R18. Fit 205/85R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,793 on the road, so it over-counts by 207. At a true 70 mph the first turns 726 times a minute, the second 742.