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ok guy here's the deal. got a 49 3600 going on a 1970 chevy 3/4 ton frame with a 472 cad and th400. ok what rearend and what ratio would you run. for the most part it will just be local truck but from time to time will have gooseneck hooked up but I would like to get out and run 60-65 and get the best mpg that's possible

With it being a '70 truck, you may have the 4.11 gears already. Otherwise, you more than likely have the 4.56.

The 4.11 would be fine. Your RPM will still wrap up more than most folkd like though.

Being you have the big block, I'd suggest a 3.73 or 3.42 rearend. The only way you will get that is to swap to a 73 or newer 14 B rearend.

The th400 is a good transmission, however first gear is fairly high and top gear is direct. What this means is the transmission was much better suited for a Cadillac than it is for a truck pulling a gooseneck.

So putting higher gears in the truck may suffer getting a load going, and keeping the 4.56 gears the truck will tach high and get poor economy on the road.

Changing transmissions may be a better option than changing rears. I would look into a built 700R4 transmission which would give you a lower first gear and an OD. I'm not sure if buick, pontiac, oldsmobile, or cadillac ever used the 700R4, so an adapter plate may be in order. I son't know what size tires you will be running, but the following table is my reccomendation for tires and gears with an OD transmission. This is not a law or anything, just a reccomendation.

28 inch tires with 3.73 gears
31 inch tires with 4.11 gears
35 inch tires with 4.56 gears

Most of these combinations will give you about 2200 rpm at 70 mph with an OD transmission. The gears or tires can be changed to push toward economy or work. These reccomendations are what I feel to be middle ground

Depending on what model '70 it is, there is a possibility it could have a Spicer/Dana 60 differential, especially if it a leaf spring chassis (usually a Camper Special). 4.10 is the common gear there, but different ratios are very easy to find for it.

ok thanks guys. sound like ill just have to get it together and go from there. the one thought I had ill run by yall is use a transfer case and then but in some high gear for runin down the highway then when I haul somethen just put it in low.

That 472 will pull anything you hook up to. In 1980, I pulled an 18-foot 3-axle trailer from California to Tennessee on I-40 with a 1970 Fleetwood Brougham. It had the 472/TH400 setup, and a pretty high-geared Cadillac rear axle. Gross weight of the car and trailer was 16,700 lbs. I made 2500 miles at 5 MPG, and negotiated a couple of major mountain ranges. No problems whatsoever out of the old Caddy, and the odometer turned over to 200,000 miles during the trip!
Jerry