I want to get an LSD for my 3.9 automatic and wondering if 4.10s will decrease MPG. I have the stock 235/75r15 and want to go 18s or 20s with 255s later. Should I get 4.10s or would 3.92s be a better choice? I get about 20 MPG on the highway and not sure about city. I have 3.55s right now.