205/65R18 vs 185/75R18
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/65R18 | 185/75R18 | Change |
|---|---|---|---|
| Overall height | 28.5" | 28.9" | +1.5 % |
| Sidewall | 5.25" | 5.46" | +4.1 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 89.5" | 90.9" | +1.5 % |
| Revs per mile | 708 | 697 | -1.5 % |
| Axle height | 14.25" | 14.46" | +5.5 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 205/65R18. Fit 185/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.4 mph | +1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,152 on the road, so it under-counts by 152. At a true 70 mph the first turns 826 times a minute, the second 813.