Road Trip can calculate the maximum distance you can travel on a single tank of fuel based on the size of your tank and your average fuel efficiency. In order for Road Trip to make this calculation you must first enter your car's fuel tank capacity, by selecting Info -> Vehicle Info -> Tank Capacity
Once the Tank Capacity has been set, you can use the Distance To Empty Calculator to estimate your distance to empty:
The Statistics screen will also display the maximum distance per tank based on your average fuel efficiency: