When I first bought my car I had the stock 305 setup, and I got an average of 22mpg. After my motor kicked the bucket I put in a new gm crate 350 with new vette injectors and prom chip. Car runs a 100 times better. But now my mileage is down to an average of 17mpg, I can get 20mpg if I go on long trips where there is no stop and go. I have noticed that the car tends to smell very rich from gas, even after it warms up. I know it has a aftermarket fuel pump that's pumping around 19psi, but shouldn't the fuel pump regs take care of that? This is the same pump that was in the car when I bought it with the 305, I didn't change anything as far as fuel pump, etc, and I got much better mileage.
What fuel pressure should the car be running at, I have heard 14psi for the cross-fires or they won't run right, is this ok? Second, what is making my mileage bad, what kinds of things should I be looking at for the problem? What kind of mileage is good for my car/setup? I would greatly appreciate any help or input. Thanks! :hail:

 

If you were getting 22mpg (about the same as me) with the 305 setup, and your STILL getting close to 20 with an extra 45 cubes, you've done about all you can. Not too many places will you see a 350 get 20-22mpg. LOL, I have a 454 that the BEST it EVER got was 16... As for the fuel smell, check all your connetions, especially at the filter. It could be as simple as tightening a few loose screws...

Bruce (90RS305)

 

That's not true. I got 24 mpg with my original 305. I replaced that w/a 350 (LO5 long block) and my milage went up to 25. I eventually replaced that engine with a 400(the engine in my sig) and it dropped all the way down to a sorry 24 mpg on road trips. lol. Somewhere in that mix I went from the stock 3.23 gear to a 3.45 as well. I have concluded that CID, and rear gear aren't NEARLY as important as state of tune.

For a mild 350, you are running WAY too much fuel pressure. Try ~14-15 PSI. I bet your milage will improve a bunch, and your power probably will too. Only way to tell is to put it on a dyno or run it at the track. Tuning it at the track is how I dial my FP and injector size in.

 

Thanks for the help and input. This really helps me with tuning my car to top performance and mpg. I will start with fuel pressure and go from there. I do plan to someday put 3.73 gears in the rear, now they are the stock 3.23. I am hoping this will help with mpg, I have been told so it will, sound true? Anyway, thanks again. :hail: