A truck with 48-in. Diameter wheels is traveling at 50 mph

First you should make sure that you are using the same units. The speed of the truck is 50 mph. Every mile is $63360$ inches. Hence

\[S=50  \text{mph}= 50\times 63360    \text{inches per hour}= \frac{50\times 63360}{60}   \text{inches per minute}.\]
Hence 
\[S=50  \text{mph}=52800   \text{inches per minute}.\]

Next we compute premeter of the wheels:

\[P=\pi r^2=\pi (\frac{48}{2})^2=\pi 24^2=1809.56   \text {inches}.\]

Now we answe the question "How many revolutions per minute do the wheels make": 

\[=\frac{S}{P}=\frac{52800}{1809.56}=29.18   \text{revolutions per minute}.\]

Next we compute the angular speed of the wheels in radians/min. Since each revolution is $2\pi$ radians and we have $29.18$ revolutions per minute. The angular speed of the wheels in radians/min is

\[29.18 \times 2\pi=58.36  \text{rad/min}.\]