I have an 86 F350 4x4 with a 6.9 Diesel and a C6 Automatic. Currently i have 37' Dick Cepeks with the stock 3.55 gears. what would be a good gearing choice for 42's?

 

You are buzzin that oil burner around 1850 rpm now at highway speeds, if you stay with the same gear and 42's you will drop the rpm to 1558 rpm. Truck will not like that.
If you install 4.88's you will bring that engine rpm to 2140 rpm at highway speeds. 5.13's would add another couple of hundred rpm.

 

with 4.88's am i still goin to be able to do 80 on the interstate? this is no trailer queen, its my DD and im in high school. forgot to mention the tach is all wacked out, so i never now what the rpms are. got a new alternator that put out more than stock, guess tach runs something off the alternator? nother story

 

i'd go 4.88's maybe even 5.13's

-cutts-

 

Like these guys said, 4.88-5.13
I'd put 4.56 or 4.88 with the 37s you have, but since it sounds like your going to go up to 42s, then at least 4.88.

 

thats what ive been told. the 37's with 3.55's isnt so bad, but what i was wondering was do gears get weaker the lower you go? someone told me anything lower than say 4.56 and you begin to shear pinion gears

 

You will probably break a u-joint before you start shearing pinion gears...

If you don't mind the 37's and 3.55's then you should probably get 4.88's.

Right now, you should be at 2095.45 RPMs going 65, but thats assuming that you have true 37's, but thats not likely.
With 42's and 4.88's going 65, you will be at 2537.60 RPMs.
Stock (33's and 3.55's) you would be at 2349.45 RPMs.

http://www.supermotors.net/vehicles/...tors/index.php

 

they are true 37's, they neasure out right at 37, they only have 250 milesw on them