I am looking to swap a later style 12 bolt irregular truck rear end under my 56 chevy, so this is fairly doable right? might have to grind off the original shackle location and weld new ones on correct? I know that they are about 1 inch wider the original, but thats no big deal, what gear ratios do you guys think would be good, I am looking for a good combination for the highway as thats where I will be doing most of my driving, but would also like to be able to up and go when needed, and not slug along, the originals back there now are like 4.11 or 3.91, so what do you guys think about 3.40's or 3.08's? i dont like the idea of 3.73's, they sound too tall and 3.55's might as we ll just be 3.42's. The guy at the classic car junk yard says he has 2.73's in his apache but I just cant see using something that tall, I mean it wouldn't be able to get out of its own way unless it has alot of torque right? by the way I am using a little 3 speed overdrive muncie, and help here would be hot, I mean it just sounds like that gear ratio out back is just screaming and it vibrates alot, I am doing this in the next two weeks, so I need some help fellas, thanks in advance.
What engine are you running? I have a 270 in my '53 pick-up. The trans is a T5 with the smaller OD. The rear end is a Dana 44 posi with 3.55. At 2700rps it's doing 75 mph and gets 16.9 mpg. I rarely need to shift out of OD. With the better OD gear set, new tires, and some tuning I think I can get 20 mpg.
In my -55 sec.ser GMC 1/2 ton truck I have swapped to a 12 bolt rear axle from a -75 4x4 Blazer. It has 3.08 gears and a locker. Truck has its original 248 and 4sp Hydramatic, and it has no problem with keeping up with traffic and cruises easy at 65-70 mph.
Only modification needed is to weld on new spring brackets. Be carefull and weld a little at atime, or the tubes will bend!! The orig brakes is a direct fit, so is the driveshaft, and it has the 6 wheel lug pattern. It is 1,5 inch wider than the stock axle, this is no problem when using stock size rims, but my 8,5 inch wide slot mags was too wide.
Look out for an axle from any year 4x4 Blazer or 4x4 K10 pickup. If you are running a trans with 1:1 high gear, 3.08 or 2.73 is fine. If your trans has overdrive high gear 3.42 is fine.
This is an easy swap that will improve yor truck for todays traffic.
FWIW I have used a program to estimate my engine speed on the freeway, I got it free at
http://www.5speeds.com/rpm.htm Do you know what the overdrive ratio is in your Muncie? Is it OE or one of the 'new' OD gear sets?
if you are running the 3 sp od. than i would say 3.55 or 3.73. if you go 3.08-3.42 it will not have any acceleration power. tom
it is the stock 3 speed od muncie; no syncro low transmission. I dont know the od ratio, but whatever the stock ratio is, thats it. uh I have a 235 right now but am going to be dropping in a stout 292 very soon, like 3-6 months, I am not looking for major acceleration in o.d., just to cruise on flat freeways. Mostly i am looking to lenghthen out all my straight gears and quiet the rear end dowm a little, the guy has 3.08 rears but this sounds too low for me, sounds like 3.40's might be the ticket huh, yeah I am not looking to climb hills in od, I can use straight third for that, which will climb at 65mph at about 3200 rpm, so the lower rearend will help that too as my motor gets great pull from 2000-3000, plus I am getting ready to load up to weber carbs to help power there too, so any other thoughts you guys have about maybe the installation or insight to good ratios. Like stated earlier though, the stock high rearend just sounds like it is going to fly apart because the rear end is so high and its spinning so much, thanks in advance.