can you fellas please give me some advice to improve gas mileage. I have a 74 convert with a 350 and TH400. I don't know the rear gear ratio but it's turnin about 2500 rpms or so at 60mph. I did a ruff estimate this mornin and i figured i'm doin between 6-8 mpg around town which is really only where i drive it. I put all new ignition system in it, long tube headers(just for kicks). the previous owners i believe put a hotter cam and definitely an edelbrock qjet replacement. the car runs and performs rather well but i don't wanna have to stop driving it because gas mileage is so bad. what would ya'll say is the best thing you can do for the money to help mileage. I've heard K&N filters, new exhaust, etc.

thanks alot,

Brad

LOL...you need to get a diferent kind of Vette if that's your worry.
Here's what you need:

http://mychevette.homestead.com/mainchevettepage.html

Dep

Something's not right. You should get nearly twice that. I'd check the timing curve (initial, max without vacuum and max with vacuum). You could have frozen advance weights for instance or a busted vacuum advance. After that look into a wideband air/fuel gauge such as innovate to help you perfect the carb jetting. 12-14 city should be easy.

I would get 18 mpg at highway speeds with my '74 L-48 auto. She'd run all day in that range when tuned.

Markus gave sound advice. Your RPM's at 60 for a T400 are about stock with the 3.08:1. You don't want to go higher for mileage.

If the 74 is mostly stock and it has been many years or an unknown time - rebuilt the carb or get an Edelbrock 600 cfm with electric choke - should get better than a Q-Jet unless the Q-Jet is well tuned for your car.

Check out the FAQ's for Lars papers on tuning, timing and Q-Jet rebuilding.

He says he already has the edelbrock carb, would be nice to know what size, 600 cfm should be plenty for a 350sb.

I also have the edelbrock 1405 600cfm carb and my milage is not great but I'm guessing about 12mpg.....

Sometimes it's a gas leak that' causing this.... 6-8mpg is way less than normal, even the big U-haul truck that I rented when I moved did better than that...

i agree with the others.....6-8 is kind of low......8-12 is probalby realistic unless its a monster motor

Install a TPI fuel injection system from an 80's Vette. You can pick them up all the time at swap meets.

I agree that you have a fuel issue. I think that you can make the greatest gains by looking at your carb, it’s the only thing that dumps fuel into the engine. Obviously you are putting to much fuel into the engine. This can be caused by a series of different situations from having rods that are stuck open to idle screw mixtures, to a sunken float. If you do an advanced search there are some great papers on troubleshooting carbs.

thanks for the help, i checked my tires today on a friends advice, wayyy too low, seems better now, should help me some, I'm gonna check into either a carb rebuild or maybe if i'm lucky one of those TPI systems.

Brad

He says it's an edlebrock Qjet, which are 790CFM

First thing you need to do is find out what the milage actually is.
I recorded every fill up and mine can swing from 6 -30 mpg. Its about 14.
A single quick check can be waaaaaaay off.

Seems to be a little big for a SB ???? My gas milage is not the greatest and I only have the 'small' 600 cfm...

Edelbrock Qjet and Edelbrock Performer are two very different carbs. A 600 cfm Peformer is equivalent to a 790 Qjet. Squarebore vs. spreadbore and all that...

That's funny !
I almost bought one of those heeps years ago.

I have dual 1405's on my 76....I get 10 mpg around town, which is where I normally drive her...'course, I'm known to put my foot into it more then I should...

I get 8.3 mpg in my 69 502 M22...mostly around town....12 in town on the 78 L-82 M20...

..think you need a good tune...

I was getting 7-8 mpg on my 76 with an Edelbrock performer carb. Did a top rebuild and now getting more power and 12-14 mpg, as high as 19 on the highway.