what rear end would get me better gas mileage with a 355 motor but still push my motor to the max

 

One or the other buddy. Unless you can fabricate a 2 speed rear diff from an industrial truck of some sort.
3.42 gears are a good choice if you have the th700r4, 3.73 if you have a 5/6 speed. The th700r4 has a really deap 1st gear so it doesn't NEED to have large gears to get out of the whole quick. I'm thinking about going wiht 3.73's with my auto. I'd be at 2500rpm at 75mph which is exactly 500rpm more than my current 3.08s.

------------------
, Jon (350 TBI!)
91 Red My website

 

I've recently installed a 3.73 rear end. I have a 350 TPI and 5-spd. Presently, I'm averaging 18.5 to 20 mpg. Not at all bad, I think....

------------------
1986 IROC-Z28 L98 TPI BW WC T-5
Air Foil
K & N
Custom Ram/Cold Air Induction
Koni Gas Shocks
Spohn LCAs and PHrod
Art Houser/Moser 3.73:1 Torsen Diff
Amsoil lubes and oil filter
Flash to Pass feature & 100w Euro lighting
Recaro Driving seat
Hurst Comp + shifter
Random Tech SHF cat