I've got a 95 K1500 350 TBI 4x4. 4l60e and 3.42 gears. I used to touch 19 mpg going 75 on the highway. I removed the stock exhaust and put on a 3 inch hi flow magnaflow catalytic converter. The muffler is a 2 1/2 inch and out straight through muffler. I have a 2 1/2 intermediate pipe and a 2 1/2 inch pipe coming out of the muffler exiting before the rear wheel. It runs strong. It idles like s... and my hwy mpg has dropped to 15. Anyone have any ideas?

Thanks,

Ryan

 

Sounds like something underhood got disturbed and
caused a vacume leak or something-the exhaust mods
shouldn't affect the idle at all.
When i had a vacume leak on my CFI z28 it did cause
poor fuel economy.