185/75R18 vs 205/70R18
Going from one to the other changes the overall height by +1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 185/75R18 | 205/70R18 | Change |
|---|---|---|---|
| Overall height | 28.9" | 29.3" | +1.3 % |
| Sidewall | 5.46" | 5.65" | +3.4 % |
| Section width | 185 mm | 205 mm | +10.8 % |
| Circumference | 90.9" | 92.0" | +1.3 % |
| Revs per mile | 697 | 688 | -1.3 % |
| Axle height | 14.46" | 14.65" | +4.8 mm |
| Approved rim width | 5–6.5" | 5.5–7.5" | 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 185/75R18. Fit 205/70R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.9 mph | +0.9 |
| 90 mph | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,129 on the road, so it under-counts by 129. At a true 70 mph the first turns 813 times a minute, the second 803.