10-12mpg with AC on, and 15-16mpg with the AC off. Does that seem right? Seems a little on the thirsty side to me. My compressor makes a nasty annoying hum in certain rpms and with the AC on, at idle the rpms surge from 600-1000 sometimes. Air is nice and cold tho. Just seeing what you guys are getting.

I concur.

I get about 10-13mpg on the streets and ab out 15-16mpg on the highway. Thats without the A/C on. I dont know about it on. Could it be that were running it too rich maybe? If i were to check codes, what codes should i be looking for. I got a freshly built L98 0.30 over and redone heads. Everything else is stock, including exhaust with dual cats. What can i do to bump my street mpg up to 15-18 lets say? I hate having to fill up every 5 days.

Could be all sorts of codes or just one. Do you have an SES light on? When was the last time you did a tuneup, properly inflated tires?

Nope, no SES light on and my tires are good. I havnt gotten it tuned sincethe rebuild but I woulda thought the shop did that. I'll call them and ask. And how do I check codes anyway? Like what do I need.

A scanner would be really nice to have as you can watch the engine parameters in real time and see whats wrong with it. Checking codes is easy tho. Take a paperclip and bend it into a "U" shape. You know where the scan port is right? Under the driver dash area kind of where your right knee would be. With the car off, Take the paperclip end and plug them into the 2 top right ports. Now turn the key to ON, not the motor, and codes will start flashing in beats through the ses light. 1 beat then 2 beats is code 12. It will flash each code 3 times per cycle. Code 12 is nothing, it says that the computer is now outputting codes basically. Depending on the codes you have, it will end on code 12 again and repeat the codes until your battery goes dead. I would have a pen and paper ready and just write down the codes as they pop up.

A shop might give you new spark plugs, but more than likely unless they are really good or it was in writing, they will just throw everything back on. Old sensors, old cap and rotor and old wires etc....So you have a new motor, but old injectors, old EGR. Replace all the old stuff with new stuff.

i suppose that could be right. i have no A/C on my car but i get about 17 MPGs in town w/ my stock 173K original engine.

running w/ it on is turning another pulley and when the clutch is engaged even harder to turn using more power. the whining from your comp could mean that its going out.

Sounds like you need a tune up. Plugs, wires, air filter, fuel filter. Check the timing. Check the fuel pressure. Get your injectors cleaned out. I got a 2.7 mpg increase on my Sunbird when I did that.

I get 20 mpg with the 5.7L in the GTA. That's on the highway. City mileage sucks. But I'm right in the middle of rush hour traffic at any time of day.

I find A/C only takes about 1 mpg compared to being off.

Really depends on what kind of driving you do. My '89 L98 will get 10 or less in stop and go-no AC- but will get 23/24 on the hiway-only drives with AC ON.

Impressive highway mpg. Do you have the 2.77 gears?

I would imagine that on the highway, it uses less gas to run the A/C than to have the windows open.

Oh yeah, hows your car now?

Yes, just the BW 2.77 rear. Personally I'd think it'd do better with 3:07...maybe 3:27...at 65-ish I'm only turning about 1800 RPM's IIRC, not quite into a good torque curve.

Might sound good, but like I mentioned before, in stop and go (Bay Area stuff) I'd would have been happy to pull 10MPG's.

Ouch I pull that in Dallas area traffic jams with the A/C blasting, pulling around a fullsize van.

I started with a bolt-on 305 TPI and swapped it for a Vortec 350 TPI and now a 383 Vortec TPI with SLP runners. Last tank was 15.4x mpg, running it HARD, I drive about 50% highway and 50% city, both in heavy traffic. I am far from a patient driver. I actually hit 20.3 mpg on a long road trip with the 383, which had less than 600 miles on it at the time.