Hi There,
my friend has done a Decat on his 3.7 I5 Hummer H3, the exhaust shop did install cat "foolers" so no check engine light errors etc, but his fuel economy has gone south. I would like to assist him with my HPtuner, but the question is where to start, and what to look for?
Read tune, disable rear O2s. You can pull the foolers if you want, but the O2s are disabled in the tune so really doesnt matter.
Clean up the MAF plus VE tables or VVE (pre 08 trucks use P12 w/multiple VE, 08+ are E67+T42 with VVE). Playing with the trans makes a big difference in how they feel, might help with the economy but that will mostly come from straightening up the fueling.