69 gearing Q? rear end req

[robbor]

I getting a hand me down 69 C10 basically stock w/ 3 speed saginaw &OD which is .85/1. The truck currently has 410s in the rear and i want better mileage for longer freeway runs hunting and fishing. It has stock skinny tires prob 28-29". Im not sure of the gear ratios in the saginaw. Anyways im wondering if i should go 3.08 or 3.42?
Sorry stock but with a new 290hp/350 crate eng.

[SS Tim]

What trans. are you running that has a .85 O/D ratio? The factory '69 OD trans. ,the M10, ran a .70 O/D with a 4.10 rear axle.

[robbor]

thought i saw somewhere online it is supposed to be .85 for saginaw ods

[SS Tim]

OK, again what trans. are you running? Is it a M10? Is it on the glove box SPID? Is it a factory installation with the the solenoids in operating condition? Are you running points in your crate engine?

[robbor]

I dont know the model of the saginaw 3 speed, factory origional. Slightly modified to use od in any gear as i recall.
Yes points, and 2bbl carb

[SS Tim]

There were conversion kits to replace the troublesome solenoids that controlled the engagement and the ignition interrupter. If it is factory and the SPID is intact, it will be there as M10.

Back to your original question. The factory only offered the O/D with 4.11s and only behind mild engines. So a 4.11 becomes a 2.88 as is. A 3.42 becomes a 2.39 and a 3.08 goes all the way down to 2.16. These seem to be way too high for even an empty truck. If you really want to try this, a 3.73 for a 2.61 might work. First thing to do is measure your tires and hook up an accurate tach. Document engine and road speeds as it is. Then try a set of really tall tires (33s ?) on the back and see what your drivabiltiy is like before you change to comparable gears.