71 rear end questions - changing gears?

[Brently]

I have a 71 with a 3:42 posi rear end in it - the clutches seem to be stuck because the left rear wheel pops in a slow turn - it seems to be to low of a gear for the 350 / 350 TH in it - although this is the rear end and motor and transmission that came in the truck - it has great take off but winds on the highway - i am very rear end ignorant - what gear ratio do i need for highway traveling that will lower the rpms and save on gas and from what vehicle year and make do i need to get it from? - HELP!! Brent

[cdowns]

overdrive tranny

[smitty62]

Suggest you check out this site. You'll need your actual tire diameter. Your road rpm can be changed significantly by tire size. Don't think a 3:42 ratio was available in '71, but it could have been changed. Actually, a 3:42 is not a bad street ratio--it's what i'm putting in mine with 235/75/15 tires. Are you sure you don't have a 3:73 rear?

http://www.quiknet.com/~ke6vut/rpmcalc.html

[Brently]

would it be beneficial to me to change the 350 turbo to a 700R and leave the 3:42 gear alone? (fix the popping in the left wheel obviously) by the way - what weight gear oil do i need to use in the rear end - i know i need to add the additive to it being a posi unit - but what weight gear oil is the best to use - i have changed it once and the popping is still there - if i use a 700R out of a 86 Astro van do i need the computer bypass that they sell to plug into the plug on the transmission? or do i need to just put it in and it will work? if i do need the plug to bypass how does that work? thanks for any help! Brent

[Tx Firefighter]

The additive should cure your popping. Pour it in and drive the truck round and round left and right a few times to disperse it to the clutch pack.

[Brently]

does the rear end gear oil matter what weight it is? if i change it and add the additive what weight should i put in with it? i have already changed it once and added additive to it and it still pops - whenever i changed it i did not work it right and left over and over - do i need to do this? thanks

[MOTHERTRUCKERS]

The GM posi additive sould work the popping out of it unless it is to far gone then a clutch/spring pak is needed. 3.42 is an awesome gear with a 700r4 if the trans is built right.

[Brently]

if this does not stop the popping, what all does a change in the spring/clutch pak consist of? can i do it? how expensive? also i am ignorant on the 700r's - i have one out of a 86 Astro van - will this fit? is the length the same and the spline of the yoke? what about the computer bypass to lock up overdrive? can you or someone please explain to me step by step what i need and exactly how to put this 700r in and wire it up correctly so it will work - it has a 350 turbo in it now - thanks for the help - Brent

[fast71]

Brently ,, Nice to see a fellow chattanooga person. I went through the same thing . I went from a 373 to a 342 and finally a 307 rear end . I 'm very happy with my choice . alot of people on here didnt want me to change the ratio to what I went to but if I had to do it all over agian I would .

[Gene]

I may be wrong, but I believe the only gears avaiable in posi in 71 were 3:07, 3:73, and 4:11. Mine is a 3:07 and I plan on installing an overdrive in mine, the rpm's at 65 are around 3300 with these gears.

[fast71]

I just changed the rear end . I can go over 200 miles 12 gallons of gas.

[68w/sbc406]

Originally Posted by fast71

I just changed the rear end . I can go over 200 miles 12 gallons of gas.

i can go 200 miles on 20 gallons