205/70R15 vs 185/75R15
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/70R15 | 185/75R15 | Change |
|---|---|---|---|
| Overall height | 26.3" | 25.9" | -1.4 % |
| Sidewall | 5.65" | 5.46" | -3.3 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 82.6" | 81.4" | -1.4 % |
| Revs per mile | 767 | 778 | +1.4 % |
| Axle height | 13.15" | 12.96" | -4.8 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 205/70R15. Fit 185/75R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,858 on the road, so it over-counts by 142. At a true 70 mph the first turns 895 times a minute, the second 908.
What drivers reported
205/70R15 carries 98 complaints. Nobody has filed one naming 185/75R15.