I'm hoping for some advice here.

For my 64 cutlass I am thinking about having a 9 inch built .
I have two questions:

axle width
I currently have 235/55/r17 tires on the back and they are mounted on a 17x8 wheel with 4 1/2" backspacing.
These do fit on the rear but have more room to the inside and a little tight towards the outer wheel arch.

I'm not sure if I will keep these wheels forever and may move back to a 15"
For the sake of future proofing I was thinking about having the axle narrowed 3/4---1" each side
This will let the current wheel fit better, and my thinking is it will give me more flexibility in the future.

Is there a downside I am missing here?

axle ratio.
That rear tire 235/55 r17 should be 27.2" tall
I have a muncie (1967) M20 which should be 2.52-1.88-1.47-1
I have been looking at at gear ratio calculators and driving myself crazy with the minutia.

Id like to have decent highway ability and just an overall fun car
Ive been going back an forth between a 3.5 and a 3.7 gear set

at 3000 rpm the 3.5 gives me 69 mph and the 3.7 gives me 65.

I feel like I should be able to cruise at 2800-3000 pretty easily for where I drive

Anyone have any real world experience or advice?

 

Matt, depending on what traction aids you have planned, the best off-the-line performance usually results when the rear-axle ratio times the first-gear ratio is about 10.

This is just another factor that you can balance when deciding.

Good luck with your project!

 

I follow you.

FWIW I am going to use a tru-trac

 

In this day and age...I would personally go with a OVERDRIVE trans. Then you get everything you want.
Again just an opinion but all my cars are overdrive except for the 1967 camaro convertible 2.73 ratio with 350th 2000 stall and 327 sbc with a 252 comp cam and 202 aluminum heads.... Wifes car never really drives hard we did track it a few times at 15 flat in the 1/4 miles goes 90 on the freeway all day long...lol
The rest are all 700R4 2000 stall v8 1980 monza 3.08 stock 270 hp budget table boat motor engine lol. Fun little car
1971 Monte 454 SS 700R4 2000 stall iron head 206 1.72 valve 781 casting performer intake 272 HMV flat tap hyd cam 750 dp holley 3.55 gears 12.70 car
1999 s10 LS1 motor stock alum 5.7 motor 212 duration com cam nothing race like... 700r4 2000 stall 12.80 at the track 3.73 rear ratio
ALL are very nice combos and enjoyable in all RPM ranges.

So really it depends on your camshaft engine profile as to where you are going to run your car at.

I have learned over years of building motors ...too big a cam for the street is just a disappointment.
the numbers on these vehicles do not sound like winners ....until you get in and drive them.... tons of low end torque on all of them that comes in quiet.

So really you can ask everyone all the questions you like...but you have to ultimately have to figure out what you want your car to do for you personally.

I am a BURN OUT GUY..... car can not do a easy 50 to 100 footer...I am going to be disappointed. top speed and track times are not as important....IN MY STREET CARS.

I have race cars. do not need a race car on the street.

I do not know if you got anything out of my post....lol. Just giving you more to stir into your pot. Jim

If you are just hanging mostly on the street and back roads up shifting and down shifting and not too much freeway....your combo should be good as long as you do not have too much duration in the cam.

 

In an ideal world I would do a 9” and. Tko

I keep think about swapping the whole drivetrain between the cutlass and the 55 truck
carbed/cammed 5.3. with a Muncie vs injected stock 6.0 and a tko

ultimately I think I’m only going to keep one

 

More info will get you better quality advice.

For instance, you say you're looking for a "fun car" that's good on the highway. Sounds like you're committed to the Muncie you have on hand so overdrive is out, unless you want to go Tremec or similar.

My questions are, what engine are you using and how hard do
you plan to run? Those old 8.2's (if in fact that's what's in it now) can take some punishment (although I don't know what aftermarket support is available these days) and the (likely) original 3.08 gear is not a bad multi-purpose ratio. You may be able to avoid the expense of building and adapting a 9" for your purposes, which will doubtless be a very spendy proposition.

Just some food for thought.

 

I was working under the impression that the 8.2 was fairly weak, but I dont know where I got that from.
Based on your replies and I have started looking into what is available.
I see Yukon makes a limited slip carrier as well as a no name clone, and I do see gearsets available from several suppliers...

I will do some reading on it. If i decide to build the 8.2 that really allows me to move forward with some other decisions which is cool..

Ive never built a rear end but I would like to know how anyway..

thanks all..