rearend gears/transmission question

[chris.c20]

i have a 77 chevy 3/4 ton camper special. 14 bolt rearend, th350. i am in the middle of building a 406 out of a 400 block, with aluminum heads, kb pistons, 5.7 rods, got a holley 750 to go on top of the rpm intake manifold im looking into. havent decided on the cam yet. getting closer to build day   

im going to be using this as a street truck, and to pull my 5th wheel so id like to save as much gas as possible, if at all possible. would a th400 would be a better choice of transmission? or rear end gear ratio upgrade? not sure of what the rear is currently but the rpm's are pretty high when cruising on the highway, so i dont like going much faster than 65-70 for cruising. would like to increase that. any thoughts or recommendations? not looking for a significant gain, but some is better than none without decreasing acceleration etc etc

[zieg85]

You really need to know what the cam specs are before you make the rear end gear choice.  You really need to know what gear you currently have.  It is going to be hard with a TH350 or TH400 to get any economy with what you want to use the truck for.  I was able to get 10 at best pulling a old school heavy 5th wheel trailer with my C20 with a 454 4 speed 3.21 rear end gear.  Got as high as 15.6 mpg not pulling anything but kept it around 60mph, speed limit was 55 at the time.

You may even consider a well built 700R4 which has a lower first gear to get the truck moving and use OD when not pulling a trailer.  My $.02

[chris.c20]

whats a good deal on a built 700r4? or the cost to have one built?

[Irish_Alley]

if you have one you can get it build prob for under a grand. BUT the amount of money you put into the trans it might take 5 years for the mpgs to pay off.
but lets say you get 8 mpgs now and you up it to 10 by better driving. with the gas prices like they are now its almost $1k year if youre driving 15,000 miles/yr

now from i understand the difference in the th350 and the th400 is the th400 is stronger but takes more hp to turn it. now will you notice it no cause its in the single numbers.

but again how long will it take the od to save you money, for the price of a 4l80e and standalone computer (91+) your probably looking at $1k-$1.5k. do some research on monster transmissions i see alot of people have them online and at the same time i see a lot of people have problems with them most are cause of kick down cable problems/adjustment and they burn up the trans.

in my impala i can get 24 mpgs going 65 or i can drop it down to 55 and get 33mpgs. i like to go a little fast, i know its not the same as our trucks but its the same ideal. the faster you go the thicker the air becomes the more resistance our trucks have to cut through eats up mpgs. lower the resistance better mpgs (i.e. dropping cruising speeds)

[Mario]

I would invest in one of those $500 all aluminum radiators because that engine will heat up fast hauling.  For that kind of displacement, I would have gone with a 396 or 402 big block.  I like the idea of getting in the high teens for mileage, so I would chose a 5/8" crank in a 350 block.  It's basically what's in the 6.0L.  I ran 4.11 gears with 235x85R16s in 1:1 at 75-80 mph, and I could hardly understand my own thoughts because the RPMs were around 4k.  Now I have an overdrive, and it's a world of difference.  I would say that the 4L80E is your best option.  Just my opinions.  I like the idea of taking the new design concepts, and applying them to our older trucks.

[chris.c20]

i had steam holes drilled into the heads when i had them built and i have an aluminum radiator, it looks about the same size so im going to try to use it. i believe its for a vette or something high performance. not sure. it didnt overheat before i took it apart and i hauled several loads over the washington passes, from puyallup to omak, and it was like 110 degrees in some parts so im not too worried about it.