165/50R15 vs 185/45R15
Going from one to the other changes the overall height by +0.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 165/50R15 | 185/45R15 | Change |
|---|---|---|---|
| Overall height | 21.5" | 21.6" | +0.3 % |
| Sidewall | 3.25" | 3.28" | +0.9 % |
| Section width | 165 mm | 185 mm | +12.1 % |
| Circumference | 67.5" | 67.7" | +0.3 % |
| Revs per mile | 938 | 936 | -0.3 % |
| Axle height | 10.75" | 10.78" | +0.8 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 165/50R15. Fit 185/45R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.1 mph | +0.1 |
| 70 mph | 70.2 mph | +0.2 |
| 90 mph | 90.2 mph | +0.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,027 on the road, so it under-counts by 27. At a true 70 mph the first turns 1095 times a minute, the second 1092.