175/80R15 vs 185/80R15
Going from one to the other changes the overall height by +2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 175/80R15 | 185/80R15 | Change |
|---|---|---|---|
| Overall height | 26.0" | 26.7" | +2.4 % |
| Sidewall | 5.51" | 5.83" | +5.7 % |
| Section width | 175 mm | 185 mm | +5.7 % |
| Circumference | 81.8" | 83.7" | +2.4 % |
| Revs per mile | 775 | 757 | -2.4 % |
| Axle height | 13.01" | 13.33" | +8.0 mm |
| Approved rim width | 5–6" | 5–6.5" | 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 175/80R15. Fit 185/80R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,242 on the road, so it under-counts by 242. At a true 70 mph the first turns 904 times a minute, the second 883.