I have an sm420 w/ 3:90 but it doesn't really get loud until 65-70mph. I can go 55-60 all day and get 14-17mpg. I have 30" tires - maybe that's the difference. I'm currently installing a gear vendors OD, because I want my truck to have the best of both worlds. It's expensive though, I had to save money for 2 years to get it. I'm shooting for comfy & quiet driving at 65 and 20mpg. P.S. I'm also getting 30" radials on 16" wheels w/stock centers.
