205/60R15 vs 185/70R15
Going from one to the other changes the overall height by +2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/60R15 | 185/70R15 | Change |
|---|---|---|---|
| Overall height | 24.7" | 25.2" | +2.1 % |
| Sidewall | 4.84" | 5.10" | +5.3 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 77.6" | 79.2" | +2.1 % |
| Revs per mile | 817 | 800 | -2.1 % |
| Axle height | 12.34" | 12.60" | +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 205/60R15. Fit 185/70R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.5 mph | +1.5 |
| 90 mph | 91.9 mph | +1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,207 on the road, so it under-counts by 207. At a true 70 mph the first turns 953 times a minute, the second 934.
What drivers reported
205/60R15 carries 32 complaints. Nobody has filed one naming 185/70R15.