Hey sorry it took so long for me to get back too you been busy. I just got back from a trip & got 20mpg on the highway at 70mph at 2000rpm. I think it's the combo of cold air intake high flow cat & muffler & 2 1/2in exhaust. I also have all synthetic oil's Mobil1 high mileage 5w30 for the motor oil. Transmission & transfer case I use Amsoil with 1 bottle of Lucius trans in it & both front & rear end's have Lucius in them. My friend has a 2001 Blazer & he gets BAD mpg like 15-16 but it's all stock & no synthetic oil's. I will say when I put my foot in it as I always do you can forget anything about MPH it's more like did you see the gas gage move. OK about the add a leaf I used them because the sagging wasn't to bad & the springs were good & they were cheap & did it all for about $60.00. I sometimes tow & I can feel the difference with them BUT if you are going to do this I would look at getting them RE-Arched it's the best bang for your buck. Hope this helped good luck