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i have a 1956 1/2 ton with a 305/350 set up with the stock rear end,390-4?? gears, i am replacing the transmission with an 700r4 and the rear end from a 86 chevy 1/2 ton, i think around 338 gears, was getting 10-12 mpg, hoping to see 15-20 when done, any one has any idea if that is wishful thing or possible

Keep the 3.90 gears. I doubt you will get better mileage with the higher gears, but that all depends on peak torque rpm of the 305. My guess is with 3.38 gears you will be lugging an underpowered motor at cruising speeds, costing you fuel economy.

You got good advice there. The 700R4 has a 30% overdrive so with 3.90 gears you have a 2.73 final gearing when in OD. Can't stand much shallower gearing than 2.73 if your ever going to have any load or pull a trailer. A vacuum gauge is great for a tell tale on gearing. If it drops to under 15 or even 16 then your geared too shallow and costing yourself fuel. If your in the 18 to 20 range your getting the most out of your set up.

thanks for the info
will get a vacuum and start checking

I am running a 350 with a 700R4 and 3:73 gears. I get an easy 20 MPG on the highway. Took a few long trips with her last summer and she seemed to stay in that range on both trips!

56srw, my street rod is a 350 sbc powered '32 Ford roadster with a mild Crane cam 218*/230* @.050 and a 2004R trans (.67 OD). It was a dog in OD even at highway speeds with 3.40 gears. With 3.80 gears it pulls all the gears well and gets a best of 20 MPG on the highway, much better than with 3.40 gears.

After 15,000 miles of test and tune I don't think I will be able to get 21 MPG out of it with my skills.

while rear end is apart doing gaskets and bearings i will go ahead with the higher gears, hard to deny 15000 miles of test and tune, after 20 years of 12 mpg average anything will be an improvement, especially since it does not run on regular gas, 305 out of a 84 z28, slight mods, on reg gas it diesels, on super reg it has slight pinging, on premium it kicks butt

Right on,with the 700R4 tranny a 3;90 rearend gear is just nice.Mine is a built 350 engine goinginto a 700R4 tranny and 3:90 rearend.Great performance and 20 MPG all day long.You can even go to a 4:11 rear and still be very economical.

thats a good looking 55

Your s.90 set up is what i am going with but i am running a 383 with vortech heads and a 625 cfm Barry Grant Carb i hope i can get 20 or better becuse of the extra tourqe the 383 makes