I get bad MPG's

[nsmall]

I will try to make this short.  I have a rebuilt 340 with J heads and some machine work to make my valves 2.02.  I hope that made sense?  The cam is bigger than stock, but not aggressive.  I have a carter AFB, aluminum intake, stock exhaust and manifold.

I also have 276 gears that is a NON sure grip.  I dont drive real slow so thats probably my main MPG  problem.  I now get 10 mpg.  I want to change my third member to a sure grip 323.  Will I notice a major drop in MPG's? I know, these cars are for fun, but gas is expensive in California, especially premium.  The guy selling me the third member thinks I may even see a MPG improvement with 323 gears as he thinks 276 gears lug the engine.  Does that sound right?

Lastly, will changing to a sure grip 323 gear third member be noticeably better when I floor it off the line than my open end 276 gears? 

I already burn the tire (note I said tire, not tires) with the 276 so Im thinking 323 sure grip will help with the traction and hopefully help with the power off the line as these 3rd members are expensive, yet I found a good deal on one.

Thanks everyone.  I tried to underline my main two questions.

[cudabob496]

3.23 sure grip would be good, it seems.
276 too low for a car you want to go fast in, occassionally.
But your cam should match the 3.23 as well.
Cam you have now may not be a good match for the 276.

[drewcrane]

I am running 3:23, 440 ,auto 12 hiway 10 city if I am mellow,8 if I am not :bigsmile:never been concerned about mileage till I did a road trip Denver to cali, fill up every 200 miles or so

[brasil]

340  x heads   727  3.23 rear      11 mpg.. 

Greetings Juergen

[Katfish]

Sounds about right

340, X-heads, 3:73 gears, 727 = 10 in town, 13 Hwy
Changed to 518 and hwy went up to 18, haven't checked in town but would guess it's 12 or so

[Mopar73340]

340 X heads 727 with gear vendors, 3.23 gears, Purple shaft 268 272 cam, air gap and Eddie 650 carb. Just started tracking mpg's but first two tanks averaged 17 mpg mixed driving keeping my foot out of it. If you want to burn the tires or stomp the gas then you need to pay the fuel bill. No way to get good fuel economy and race around. 

[stinger]

440 with Lunati 703 cam auto with 3:91  I get about 10 minutes per gallon   

[cudabob496]

vacuum advance is good for about 3 mpg

[blown motor]

340 .060 over, TTI headers, 2.5" exhaust, X heads, 650 Eddy, Lunati cam = 13 MPG

[dfrazz]

340 6-bbl 727 3.55 gears 15 mpg - one of the benefits of a 6-bbl is you are running just on the center 2 bbls most of the time.

[nsmall]

Thanks everyone.  I guess I need to drive real nice and easy to see what happens.  I really like flooring it.  I also drive in town (lots of stop lights) a lot, I let the car warm up, and never go very far.  I hope 323 gears dont hurt my mpg's too much?  Its weird as some people seem to be getting like 15 mpgs and their engine seems more built than mine. My car seems to run great.  The carb was rebuilt and everything seems to run excellent.

I really appreciate your advice.  I believe I have vacuum advance hooked up....Is this hooked up to the distributor to the carb?  If so I have this.  Thanks

[js29no]

How fresh is your engine!! If it was just rebuilt, what will make A difference. as they break in there is less friction, MPG gets better.

[Chryco Psycho]

10 MPG is low tune the crap out of it , 3.23 will help & a lead foot will hurt

[734406pk]

 
You have a lot going on there! I would change the rear axle ratio to 3.23:1 first and get an AFR gauge installed when the budget allows. 

[1 Wild R/T]

My 383 typically pulls down 12-13 MPG's driving normally not easy, not overly aggressive... Once I did a max economy run from LA to Central California (55 MPH easy on the throttle, no passing) & got 15.4 MPG's but I was bored to death.... I'd rather buy more fuel & have fun...