205/85R15 vs 205/80R15
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/85R15 | 205/80R15 | Change |
|---|---|---|---|
| Overall height | 28.7" | 27.9" | -2.8 % |
| Sidewall | 6.86" | 6.46" | -5.9 % |
| Section width | 205 mm | 205 mm | 0.0 % |
| Circumference | 90.2" | 87.7" | -2.8 % |
| Revs per mile | 702 | 723 | +2.8 % |
| Axle height | 14.36" | 13.96" | -10.3 mm |
| Approved rim width | 5.5–7.5" | 5.5–7.5" | overlap 5.5–7.5" |
Both are approved on a rim 5.5" to 7.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 205/85R15. Fit 205/80R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,719 on the road, so it over-counts by 281. At a true 70 mph the first turns 819 times a minute, the second 843.