165/50R16 vs 185/40R16
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 165/50R16 | 185/40R16 | Change |
|---|---|---|---|
| Overall height | 22.5" | 21.8" | -3.0 % |
| Sidewall | 3.25" | 2.91" | -10.3 % |
| Section width | 165 mm | 185 mm | +12.1 % |
| Circumference | 70.7" | 68.6" | -3.0 % |
| Revs per mile | 897 | 924 | +3.0 % |
| Axle height | 11.25" | 10.91" | -8.5 mm |
| Approved rim width | 4.5–6" | 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 165/50R16. Fit 185/40R16 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,702 on the road, so it over-counts by 298. At a true 70 mph the first turns 1046 times a minute, the second 1078.