I have a Magnaflow 20" muffler on the stock system. Magnaflow--who also builds full stainless exhausts, said the muffler was 75% of the restriction. I didn't have gauges until after installed the muffler, but there is a guy who installed this same muffler o his Dmax and reported a 150ish degree drop in EGTs. I guess the full 4" exhaust would be worth another 50 degrees or so, but I have't seen any side by side tests.
The low ET guy I provided the link to says in his signature that he's running a Crew cab 4x4. I don't know if he is launching in 4WD or is using slicks in 2WD.
Blaine
