Sound Deadening Advice

I put in over 100 pounds of sound deadener, and 50 pounds of additional audio equiptment - no MPG hit at all, I can still pull 60's and 70's on some flat 45mph roads around town, I get 55MPG on 55MPH highway stretches, and mid to low 40's when on high speed interstates for a distance.

Added benefit is the insulating effect of the material as well - I did notice that both my A/C and Heat worked less after the install, it adds thermal insulation, and taking load from the HVAC system would actually have a positive benefit on the efficiency of the vehicle - so it may in fact have a positive effect on MPG depending on what your HVAC preferances are.