185/85R15 vs 205/80R15
Going from one to the other changes the overall height by +1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 185/85R15 | 205/80R15 | Change |
|---|---|---|---|
| Overall height | 27.4" | 27.9" | +1.9 % |
| Sidewall | 6.19" | 6.46" | +4.3 % |
| Section width | 185 mm | 205 mm | +10.8 % |
| Circumference | 86.0" | 87.7" | +1.9 % |
| Revs per mile | 737 | 723 | -1.9 % |
| Axle height | 13.69" | 13.96" | +6.8 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 185/85R15. Fit 205/80R15 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 | 71.4 mph | +1.4 |
| 90 mph | 91.7 mph | +1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,194 on the road, so it under-counts by 194. At a true 70 mph the first turns 859 times a minute, the second 843.