185/85R15 vs 185/80R15
Going from one to the other changes the overall height by -2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 185/85R15 | 185/80R15 | Change |
|---|---|---|---|
| Overall height | 27.4" | 26.7" | -2.7 % |
| Sidewall | 6.19" | 5.83" | -5.9 % |
| Section width | 185 mm | 185 mm | 0.0 % |
| Circumference | 86.0" | 83.7" | -2.7 % |
| Revs per mile | 737 | 757 | +2.7 % |
| Axle height | 13.69" | 13.33" | -9.3 mm |
| Approved rim width | 5–6.5" | 5–6.5" | overlap 5–6.5" |
Both are approved on a rim 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 185/80R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,734 on the road, so it over-counts by 266. At a true 70 mph the first turns 859 times a minute, the second 883.