205/60R18 vs 185/70R18
Going from one to the other changes the overall height by +1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/60R18 | 185/70R18 | Change |
|---|---|---|---|
| Overall height | 27.7" | 28.2" | +1.8 % |
| Sidewall | 4.84" | 5.10" | +5.3 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 87.0" | 88.6" | +1.8 % |
| Revs per mile | 728 | 715 | -1.8 % |
| Axle height | 13.84" | 14.10" | +6.5 mm |
| Approved rim width | 5.5–7.5" | 5–6.5" | overlap 5.5–6.5" |
Both are approved on a rim 5.5" to 6.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 205/60R18. Fit 185/70R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.3 mph | +1.3 |
| 90 mph | 91.7 mph | +1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,185 on the road, so it under-counts by 185. At a true 70 mph the first turns 850 times a minute, the second 834.