I just came back from the second of two trips to Chicago. I checked the fuel economy and I was running close to 23 mpg. The changes to my exhaust between the two trips are as follows.

I added 2 x 2.25" 12" long CATCO cats, MSDS headers, and different style mufflers. The HP definately felt lower when I changed the exhaust, but the new mufflers should have flowed better. I had something close to 165 on the dyno with true duals, MAFS, headers, & KKM. I got it optiminzed to 177 with a few more tweaks.

Before the change, I dynoed at about 170 with only the catless SHOShop y-pipe (no true duals), MAFS, & KKM. I also got 30 mpg to Chicago...easily.

I am not sure if there is a correlation here, but if I lose 7 mpg, can that all be attributed to the cats or am just more inefficient now somehow? Also, is there any correlation between lower fuel econ and hp?

Thanks! confused

98 Black E0 #946 of 6535
Koni struts & Eibach springs
24mm Aussiebar
Pro Flow 75mm MAFS calibrated to KKM filter
Wilwood Brake Kit with cross-drilled rotors
Clutchmaster's Stage 1 Clutch & Fidanza Flywheel
Custom Dual Exhaust with an Xpipe
MSDS Headers
Custom Cold Air Intake with KKM filter
Dyno'd at 177 hp and 156 ft-lbs