185/80R18 vs 185/75R18
Going from one to the other changes the overall height by -2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 185/80R18 | 185/75R18 | Change |
|---|---|---|---|
| Overall height | 29.7" | 28.9" | -2.5 % |
| Sidewall | 5.83" | 5.46" | -6.3 % |
| Section width | 185 mm | 185 mm | 0.0 % |
| Circumference | 93.2" | 90.9" | -2.5 % |
| Revs per mile | 680 | 697 | +2.5 % |
| Axle height | 14.83" | 14.46" | -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 185/80R18. Fit 185/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.8 mph | -2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,754 on the road, so it over-counts by 246. At a true 70 mph the first turns 793 times a minute, the second 813.
What drivers reported
185/80R18 carries 2 complaints. Nobody has filed one naming 185/75R18.