First, we need to calculate how much money Mr. Miller has left after spending $13.25 on lunch.
Starting amount: $50
Amount spent on lunch: $13.25
Remaining money: \[ 50 - 13.25 = 36.75 \]
Now, Mr. Miller spends the remaining $36.75 on gas that costs $3.75 per gallon. We can calculate the number of gallons of gas he buys by dividing the amount spent on gas by the cost per gallon.
\[ \text{Gallons of gas} = \frac{\text{Amount spent on gas}}{\text{Cost per gallon}} = \frac{36.75}{3.75} \]
Now, performing the division: \[ \frac{36.75}{3.75} = 9.8 \]
To the nearest tenth of a gallon, Mr. Miller buys 9.8 gallons of gas.
Thus, the answer is 9.8.