I have a fiber glass lid on my 04 quad cab 4x4 hemi and except for looking good I'd say it does nothing for the mileage,try 40 #s of air in your tires ,that the best advice I can give you. And slow down.

 

I have an extang and I saw about 1 mpg improvement at highway speeds. I have no lock on mine. Its more out of site out of mind deal. Soft covers still allow you to utilize your bed more with a quick roll up. Hard covers and caps essentially turn your truck into a station wagon. There are numerous options in the soft catagory and even lockable. With the price of a hard toneau, I would look into a cap.