Its funny I started a thread about this a few months ago when I got my car with the new engine in it. My car was TERRIBLE on gas, and still is as far as I'm concerned. If an LS1 Camaro gets better gas mileage than it and has 2 liters more displacement and its romped on alot, something is still wrong with my car. I went through the works like everybody has told you to do on here. I did the fuel filter, which started a fuel leak in the pressure line when I disturbed the quick disconnects while messing with it (now fixed), bought a Denso O2 sensor from Rock Auto for $40 shipped, flushed my trans fluid to increase the reliability and efficiency of my transmission, gutted my airbox, increased air pressure in my tires, and IM STILL GETTING 14 MPG WITH 25-50% HIGHWAY. I don't beat on my car at all, I have my Camaro for driving fast. I happen to be a valet parking coordinator for a few restaurants, so I stumble across my share of L36 equipped H-bodies that have the Gas Mileage analyzer buttons on the dash. I check every car that I can, and I see most H-body Park Avenues, Bonnevilles, and Le Sabres getting low 20s on the display, and I do see my share of 14-16 mpgs on the display of these cars as well. I guess I am going to chalk it up to the fact that my car just sucks on gas. I am going to replace my air filter, but I won't expect that to shoot me up to 20+ mpgs. I just can't believe some people rave about the gas mileage in these cars. Good luck with your problem because I hear gas is gonna hit $4/gal this summer.

Most cars here hit 28-32 mpg on the freeway.

I suggest you do a compression test or attend a meet soon if yours doesn't improve. The team approach may work better.

Are you using a 195 thermostat or a 160?

195 for gas mileage...I get 27-28 highway, 217K miles on the clock

Using a 195 t-stat and I just ordered a denso O2 sensor from rockauto, this is the recommended one, correct? Thanks Bryan

AC Delco is preferred. Denso is probably ok, but not proven by any kind of margin.

Mustang Man,
You are probably talkin about the one that I bought. I don't really have problems with it, its a nice unit. I probably should have mentioned that there is a code from this website that you can put in the promotional box when ordering from rock auto. This code gives you a 5% discount on most parts, it saved me $2 from the purchase. Sorry, I didnt think you were that serious on buying a denso from rock auto.

You could also check the TPS voltage at the idle position with a voltmeter.
They are sometimes not exact and could use tweaking. I know they are plug and play but you could file alittle of the holes to get slight movement.
I tweaked my 96 SE and it improved the shift points and MPG'*.