I have a 01 DTS with 42,000 miles on it and I am getting what I think is poor gas mileage. I get 14 city and 18 Highway. My father in-law has an 02 base Deville and he get over 30mpg going 75 on the interstate. The bearings and the rods and pistons or what they called a lowerend rebuild was done on the car when it had 32,000 miles on it because I noticed a knock in the morning durning a cold start. I had the transmission checked and every thing looks great. Obviously, I have changed fluids and filters during my routine maintenance. Does anyone have a suggestion? Is there a performance chip or a company that remaps the ECU? can I find an after-market air intake or any other performance parts for this car? Also, how difficult is it to replace the hvac blend door or blend door motor. Local mechanic said it would take 12 hours to replace the $100 part. Thank you in advance for any help or suggestions.