205/65R18 vs 185/70R18
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/65R18 | 185/70R18 | Change |
|---|---|---|---|
| Overall height | 28.5" | 28.2" | -1.0 % |
| Sidewall | 5.25" | 5.10" | -2.8 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 89.5" | 88.6" | -1.0 % |
| Revs per mile | 708 | 715 | +1.0 % |
| Axle height | 14.25" | 14.10" | -3.8 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 205/65R18. Fit 185/70R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,896 on the road, so it over-counts by 104. At a true 70 mph the first turns 826 times a minute, the second 834.