No, this post is NOT about heinies. I opened up the original stock peg leg 10 bolt rear in my '65 tonight. The car has a 350/330 crate engine with a 700R4 overdrive tranny. The current axle ratio is 3.36 and I am thinking of going for 3.73 with a posi carrier. The axles are pitted and probably should be replaced, the ring gear is the smaller 8 1/2" unit, the pinion bearing is shot, and I was going to go for a total rebuild. My question would be by the time I buy a set of decent axles, the bearings, gears, carrier, etc. would I be better off just going for a complete Currie 9" Ford rear all set up with the disc brakes; or something else. Any suggestions would be appreciated. Happy New Years, Everyone!

This one is a marginal one to call. But, based on your info, here is the way I'd go, ESPECIALLY if it is the rear axle housing for your car. First of all, you have a small block and an automatic. The 10bolt (and if it is the original rear, it is a 10.2), if properly rebuilt, will hold up just fine. You can replace the axles, or, you can install axle saver bearings. Some folks do not care for axle saver bearings. I've installed many, and have them in 2 of my cars and have NEVER had or seen a problem with them. If you are going to do ALL your driving in town or town and 1/4mi, then a 3.73 would be a good gear. BUT, if this car is going to see a fair amount of hiway driving, that 3.73 is going to get old in a hurry. Therefore, I'd stay with a 3.08 or 3.31, or no more than a 3.55.

Even with the overdrive, Tom?

Bowtie Overdrives has a neat RPM calculator that you can use to help make your decision. Just punch in your current tire diameter and your 2 gear ratios and compare the results. You can get to it here (http://www.2004r.com/speedoCalc/rpmcalc.html). The Currie 9" is nice, but it's probably more expensive than just rebuilding your current one.

Jeff,
If the overdrive is working as it should (I think on the 700R4 it's about .80-.75), that should drop the final ratio down to about 3.00 or 2.80 with the 3.73 gear. For the hiway, that would be a very good final ratio. Otherwise, 3.73 is going to be a little bit too low for regular hiway driving.
I have to agree with 72Sprint that it would most likely be cheaper to rebuild the original 10bolt rather than buy something as a Currie. Also, in the future, parts for a stock type rear end will more likely be easier (and cheaper) to replace than custom aftermarket parts. Plus, if there should ever come a time to sell the car, it may be better to have a properly rebuilt rear end in it. This is only for a street/performance driver, for a serious race car, all the rules change.

If the overdrive is working as it should (I think on the 700R4 it's about .80-.75), that should drop the final ratio down to about 3.00 or 2.80 with the 3.73 gear. For the hiway, that would be a very good final ratio. Otherwise, 3.73 is going to be a little bit too low for regular hiway driving. Tom, my car is a wagon, so it is a little on the heavy side. Overdrive works fine and on the highway with the 3.36's it's a little sluggish on the hills, but the highway fuel mileage is pretty good. I use this car for cruising with an equal mix of highway and 'round town driving. So what is your final verdict on the ratios? I have decided to keep the original 10 bolt and rebuild it.

Tom, are you out there?

ttt

Jim,
With a 3.36 rear and an OD tranny (.80 to .75), your final drive will be in the range of 2.80 to 2.50. For interstate type driving conditions, this is certainly going to provide excellent fuel economy. And yes, up hill may result in some sluggishness, but that is to be expected with those kind of final drive ratios. With the 3.73 and OD, as I mentioned earlier, your final drive is going to be in the range of 3.00. A car with a 3.08 gear (and a tranny with 1:1 final drive), is going to result in around 3000rpm at about 75mph. That's what I have in my 70 conv and my 56 Vette and my 51 Chevy. I have a 2.73 rear to rebuild for the 70, which should give better fuel economy, but if I don't like it for town driving and cruising, I'll put the 3.08 back in. Sometimes compromises are difficult to determine where to exactly draw the line.
Now, you wanted me to look at this again. OK, here is my honest opinion. If this is going to be a true cruiser, I'd stay with the 3.36. It may (and may not) be very responsive out of the hole (depending on 1st gear of the 700R4), but, I would truly expect that after 20mph at full throttle, it will be VERY strong). If you do go with the 3.73, and your interstate driving rpm is around 3000, it isn't going to hurt a thing. A well built 350 (or fresh crate 350) should last for many, many years at 3000rpm. I drive my 51 Chevy EVERY day to work on the interstate at 75-80mph and I built that 350 in 1973 and haven't opened it up since. Yes, with a rear gear such as 2.73 or even 2.56, it would probably get better gas mileage, but the additional improvement would probably not be that much. I like the all around performance with the 3.08 just as it is. Once I get moving over 20mph in 1st gear, and then wind it up in 1-2-3, it's going much faster than what is legal. So, when I hit 4th, and drop down to 70, it cruises just fine. I think that if it had a 2.73 or 2.56, I'd be in 2nd or 3rd more than I'd like in town.
So, bottom line, if it were mine, I think I'd keep the 3.36 and rebuild the original 10bolt.
If by chance you should EVER run across a good deal on a 12bolt 3.31, grab it and put it in.
Keep the wagon, I CANNOT tell you how much I wish I still had my 72 wagon which I ordered new (with BB, 4sp, full pwr and air).

Tom, thank you so much for the reply. Your advice is sound, and I think I will follow it. A good compromise, though might be those 3.55's. I still have some time to think it over. I was going to go with the Eaton posi and new axles, bearings, etc. Thanks again!

700R4's have a .7:1 overdrive, so those 3.36's would be like having a 2.35:1 rearend ratio.
(just a note, the 200R4 has a .67:1 overdrive)

If a 700R4 has a .7 OD, then that is correct, with a 3.36, you would end up with a final of 2.35.
With a 3.55, you would end up a final of 2.48.
And with a 3.73, you would end up with a final of 2.61.
My Cutlass has a 2.41 final, but it also has a .030 over 455, so it has the torque to turn a 2.41 on the hiway and it is great. But when using it to tow the 70 Conv on an open trailer, I wish it had the 2.73 in it (and I have one to build for it).
So, with a fresh 350 in your wagon, and a .70 OD, it just may be good to go with the 3.73-----------------------------IF YOU EVER PLAN TO TOW WITH IT. Otherwise, the 3.55 may be the better choice. BUT, with a .70 OD, that 3.36 MAY just be a little bit to much.