Rear End Choices

[azmalibuwagon]

Trying to balance performance with gas mileage, and would like to get some input from you guys who have experience with this.

Originally, my DD wagon had a TH350 transmission, but now has a 2004R . The code stamped on the rear axle housing is 8AY, and looking at the axle codes on the maliburacing forum, I believe my wagon has a 2.56 rear end, all this driven by a little 305 HO. It probably worked ok with the TH350, but the overdrive gearing just seems too tall now, have to downshift to pick up any speed at all on the Interstate.

So I want to pick up some cruising power in the 40-70 mph range, but don't want to end up with 10 mpg (right now I get about 17).

What would be a good choice? Would a 3.08 be enough, or even noticeable? Or do I need to go a 3.28, or even higher?

Any suggestions are appreciated!

 

[Blake442]

With a 200-4R, I'd go 3.42 or 3.73.

My 442 with 3.73's averaged 17 mpg around town and got as good as 19 mpg on the highway with the 307 and later with a 350 when I still had the 200-4R in it.
That is if I kept my foot out of it...

 

[pontiacgp]

with a 305 and a 200 r4 I started out with a 2:41 open diff then went to a 3:42 posi and then a 3:08 posi.....I found the rpm with the 3:42 too high for my liking so I went with the 3:08 and I found that ratio great. I now have a 355 vortec with a T5 WC and still running the 3:08...in 5th with a .68 drive gear I'm reving 2000 at 75 mph and the car still takes off the line pretty good

 

[DRIVEN]

Try playing with this calculator. Best I've found so far:
http://www.angelfire.com/fl/procrastination/rear.html
My '79 Cutlass with stock 305, TH200 and 2.29 gets right at 20mpg. Theoretically it will get the same with a 200-4R and 3.42s. The cruise RPM is almost identical with the advantage of a much better start gearing which should actually improve in-town mpg.

 

[Clutch_Grabwell]

You get to decide what you want for a final drive ratio after doing the math: (rear end gear) x (OD ratio) = final drive ratio.

2.56 x 0.67 = 1.72 (no wonder you want new gears...)
3.08 x 0.67 = 2.06
3.23 x 0.67 = 2.16
3.42 x 0.67 = 2.29
3.73 x 0.67 = 2.50 This was the stock gearing on my SS and I thought the revs were just a little higher than necessary.

 

[azmalibuwagon]

Thanks, guys, this helps a lot.

I think I'll see if I can find a 3.42 or 3.73 around here for a decent price and just try it out. Should be able to fined a non-posi rear end for a cheap price. I get the feeling that just about anything would be better than what I've got right now.

 

[azmalibuwagon]

It was over a year ago that I asked for some help choosing a rear end, and got a lot of help from you guys.

Anyhow, it took awhile but I finally put in a 3.73 rear end out of a Monte SS a couple weeks ago so decided I would put in some thoughts about it.

Replaced a 2.56. Took me about 3-4 hours and was pretty straight forward. Damn thing is heavy for one guy to deal with, though. Now I gotta figure out what to do with the 2.56...

The wagon now revs about 2500 or so at 75mph, used to rev around 1800. It feels like the revs are too high at that speed, but I may get used to it.

The 3.73 is a real hoot to drive, it really woke up the little 305, got it running in a much better power range. I don't know what this will do the mileage, having too much fun with it right now. I got 17-19 before, it'll surprise the heck out of me if it gets anywhere near that now.

It changed a couple of things that I didn't expect. 1) the car revs a lot better on the top end now. Could hardly get it over 4500 before, it climbed to 5500 in 2nd gear a couple days ago before I backed off. 2) The transmission (200r4) shifts through the gears really fast now unless I shift manually. Will have to figure out what to do about this, it's a bit of an aggravation in stop and go traffic. 3) had to adjust the tv cable to keep it from downshifting too quickly.

So far,it's fun, but I have a few concerns. We'll see how I feel after a few months with it.