OK so when it comes to a rear differential I know its called a pumpkin and that you always want posi. I need to be able to drive my car to New Hampshire and back comfortably, and just picked up a 12 bolt posi w/3.93 gears from a Monti. Will this gear ratio be too high for my needs and how different is it going to be with posi, my car currently is a one wheel wonder.
Comfortable is a relative term. You'd likely be driving right at 60mph on the highway unless you have an overdrive. I'm going to 3.89 gears shortly, but I have a deep overdrive.
Good to see Wisconsin on the board! I'm 20 minutes north of you. Anyways... I run from Appleton to Madison with 3.42 gears and a th-350, and I'm almost wishing I had 3.08 or 3.23's. Too many Rpms for too long. With the torque of a Buick there's no need to use steep gears unless it a track car. I recommend 3.08's for you. New Hampshire is a haul from here, and you'll get tired of 55 MPH with that poor engine screaming for over 1000 miles. Not to mention fuel prices. Just my .02 Are you going to any shows locally this summer?
I might do that, she does look nice right now and can go pretty much anywhere. I need to find out how to get an OD th400 built right, I admit I don't know much about the trannys either.
3.08 seems the best overall gear set for combined driving. I'm still running 2.56 in mine & see under 2000 RPM on the highway @ 60 MPH.
If you do any highway driving at all, Mark has got it right. I've got a 3.08 car that is a highway star, and a 3.42 car thats just at the edge of reasonable for frequent highway use. I also have a 3.23 car and a 3.31 car. I'd say my favorite is the 3.23 - car jumps, but still gets 16-17 mpg.
I have a switch pitch 400 with a Gear Vendors overdrive. I have a 3.73 rear with a 28" tire (P275/60R-15). That makes the rear more like 3.55. The .78 Overdrive brings that down to 2.76. 60 MPH is about 2100 RPM. You can add a GV to any 400 transmission, or any other transmission for that matter. It is rated for 1200 HP. It is expensive though, about 3000.00 when all is said and done. That is about what you will spend for a 2004R to make it live behind a BBB.
I added a 3.23 posi to my Buick with a 350 & 350 turbo tranny. Nice take off and runs at 3,000rpm at 70-75mph on the hwy. I believe it is around 2,500rpm at 60mph.
What mods are required to the original TH400 to install the OD? I've been very tempted to add one to mine but don't want to touch the original BB trans. I might just settle for taller tires in the rear and live with never touching the highway.
No mods really. The extension housing is unbolted after removing the drive shaft. Then an adapter bolts on, and the GV unit bolts to that. The drive shaft has to be shortened by about 14" if my memory is correct. That's it. Read all about it here: http://www.gearvendors.com/ http://www.gearvendors.com/magazines.html
Why don't you sell the 12 bolt rear and buy a possi unit for your stock rear end and leave the rear end ratio alone? The rear end in your 72 is a good one, no need to swap out the entire rear end!
Ok, I can add posi to my stock rear end that would be fine. How do I determine what gears are in my pumpkin right now, with out taking off the cover? Thanks for the advice btw on keeping the rear end the same.
There are ways to determine the rear end ratio by comparing the amount of turns on the wheel to the driveshaft with the rear wheels raised off the ground but is about as easy as removing the rear cover. If you have the rear cover removed you can get the numbers off the rear gears and get a positive id on the ratio you have now and go from there. Then post the numbers you find on the gears and we can help you further.
Sean is right, I put an Auburn diff in my car to convert to limited slip (or was it an Eaton?) Either way, it's not too bad a job, but requires some specilized tools - dial indicator, fixture plate and micrometer, etc...all can be had at Harbor Frieght for not too much money. And if that car truly has 3.91 gears in it, I'd be willing to bet you find you already have a positrac unit in there, just probably worn out.
No my car does not have the 12bolt under it right now. Its just a read end that was made available to me and I wanted to know if it would be good for what I want my ride to be. I still want POSI, I just need to know if I can buy it and install it in my pumpkin with some slightly higher gears now. Most likely a 3.23 would be fine but I don't know what in it stock. I'll take the car in Monday and have the rear diff looked at as I don't want to jack it up in the garage with a storm coming tonight, will put it in the storage garage for now.
Good to find some local people on the board! Maybe we'll cross paths at Pierce park this year?? Anyway, I am considering gears also. Currently running a 200-4r so nice .67 overdrive. I have the single wheel peel of 2.78's. They aren't too bad, but I know posi is a must so I have been looking for some 3.23 rears or 3.36 even. Seems like 3.23 are more common and wouldn't kill my awesome mileage too much. I was really wanting 3.55's but after reading about a good compromise gear as I like to cruise the highways, I'm back down to 3.36 or 3.23. Let us know what you do! ---------- Post added at 09:47 PM ---------- Previous post was at 09:45 PM ---------- I did see and add on clist for a 3.08 posi not too long ago for under 300.00
