165/85R15 vs 175/80R15
Going from one to the other changes the overall height by -0.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 165/85R15 | 175/80R15 | Change |
|---|---|---|---|
| Overall height | 26.0" | 26.0" | -0.1 % |
| Sidewall | 5.52" | 5.51" | -0.2 % |
| Section width | 165 mm | 175 mm | +6.1 % |
| Circumference | 81.8" | 81.8" | -0.1 % |
| Revs per mile | 774 | 775 | +0.1 % |
| Axle height | 13.02" | 13.01" | -0.3 mm |
| Approved rim width | 4.5–6" | 5–6" | overlap 5–6" |
Both are approved on a rim 5" to 6" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 165/85R15. Fit 175/80R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | -0.0 |
| 50 mph | 50.0 mph | -0.0 |
| 70 mph | 69.9 mph | -0.1 |
| 90 mph | 89.9 mph | -0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,992 on the road, so it over-counts by 8. At a true 70 mph the first turns 903 times a minute, the second 904.