^^^^Agreed. Make sure everything works like it's supposed to first. Make sure the fuel system is supplying adequate, clean fuel. Check and change the filter if needed, and clean the pre-filter. Breathing in and out is important, a BHAF wouldn't hurt. Also if your exhaust is in bad shape you might pick up a little mileage with a 4 inch system. I have found that once I installed my gauges my mileage went up some, because I started driving different. You want to keep your EGTs and boost both as low as possible for best mileage. Low temps mean clean, efficient combustion and low load. Low boost means low load and low drive pressure. Timing will also help.
98 12 Valve, 5 Speed, 3.55 Gears, 35s, 4K Gov Springs, Timed 22 Degrees, 5x16s, 024 DVs, Homemade Fuel System, ARP Studs, Ported Head, 60LB VS, 64/64/9 HE351, Valair DD, 5 Inch Exhaust. 442hp 846tq 10/11/14 5x12s and 60mm