I have a 37 chevy p/u with a stock 57 truck 235 hooked to a T-5 and had a 8 inch rear end with 2.79 gears and it got better gas mileage when it had the original 4.10 gears,what can I do to make it to run the best and get better mileage.

Keep it in third gear!

Overdrive with 2.79 gears is for land speed record racing, it will take way more horsepower then you have to pull that gearing. My '37 Chevrolet pick up with a stock 250 (140 HP) T350 automatic and 3.08 axle will run 70 mph and get around 15 to 16 mpg depending on how hard and which direction the wind is blowing. Switching from a 3.42 to 3.08 didn't change the mileage much, but it did lower the rpm. Slowing down helps, these trucks push a bunch of air with flat windshields. I can get 19 mpg if I keep it at 55.

1958 235 Chevrolet's, net hp 125@4000rpm, net torque 195@2000rpm
http://chevy.oldcarmanualproject.com/chevyresto/58t102.htm
Gear it to match the torque, 3.37 or 3.91.
Joe

With the gears that are in it I dont use 5th and I plan of stepping up the gears to around 3.55,would it help to change the cam to a RV style grind or finding another carb designed for a newer 6.

What size rear tire are you running?

The stock cam will likely give the best economy, more compression would help (as long as you don't run into detonation).

One of the Carter/Webber carbs like Langdon sells should help as well.