Gear ratio preference with overdrive
 

Prepping to LS swap (Yes, a sin I know :)) my 66 Catalina Wagon this winter. This car still has the factory rear end (posi) in it with factory gears (2.56?). This will be a mild 6.0LS with headers, intake and a truck style cam for lower end torque, not peak horsepower. I only cruise the car. It will have a 4l80e transmission. I considered putting a lower gear ratio in the car to help more with low end (have to be able to smoke the tires :)).

I know a lot of guys on here like the 3.08's but if you were going to have overdrive, would you push that to a 3.42? Just curious what those with overdrive have been satisfied with. I'll be honest, it's been a while since I've checked parts availability on the stock rear ends, so maybe I'll be surprised by what's out there.

Assuming the smoking tires are 26" in diameter, 3.43 rear gears and a max engine RPM of 5000, max MPH in gears are 46, 76, 113 and 151. 60 MPH cruise in 4th gear would be at 2000 RPM.

You will find very little available for parts for that rear. Fabcraft used to list stuff but not sure they still do.
I’d say if that LS can’t bake the tires with 2.56’s, you’ll need to build a torque monster Pontiac motor!

For a Chevy motor my default is 3.73 with overdrive; 3.42 for non overdrive application.

Nice balance between off the line performance and fuel economy.

K

I agree with Keith, I prefer at least 3.73's with a 0.7 OD if you are closer to a 0.5 OD I like at least a 3.90 gear. I do agree with the 1800-2000 rpm for final cruising. I had bought a 69 Tempest that the PO had swapped out the original ST300 for a 2004R but kept the original 3.08 rear gears, I had to drive 85mph+ to use 4th gear on it as the worm 350 2bbl did not have enough torque to keep cruising speed at 1200 rpm. It did have 16 wheels swapped on it. I can;t remember how tall the tires were though.

I was worried the aftermarket support might be little to none by this point. I guess I could do an axle swap if needed, but that opens a whole new can of worms! You'll probably call me insane, but I recently pulled a 461 out of the car and sold it to allow for me to LS swap it!!:)

I'm running aftermarket 17" wheels. If I remember the tire size correctly, they are about 27.13" tall.

The 3:42 gear set is fairly rare, but it is out there. Mostly found in 2+2 cars. they come up for sale every so oven, but they will be 50+ years old used parts. Myself I would have kept the 461 and gone ahead and added the 4L80E. A 2:93 or 2:73 would have been ok with the low down torque of the pontiac.

Just multiply each MPH value I posted by 1.043461538461538 to figure speed at the same RPM.

I went 3.08 initially on my LS3 swap with 4L70E. Swapped in 3.73s and I like them much better. It’s a little louder on the freeway but more fun everywhere else.

In the '65 Cat wagon we have a set of Fabcraft 3.55 gears, 4L80e trans, and 235/55R17 tires which are about 27-1/4" tall. Works great for keeping up with the soccer moms on the freeway.

I have 3:42's with a 4L80E and 28" tall tires and I end up doing 65mph at 2000rpm. On the highway doing 70-75 I got 15+ mpg with a 571 cu. I was happy with that.

So I called Fabcraft today because I've heard of them being a supplier (and about the only one) of aftermarket gear sets/parts for these 10 bolt Pontiac rear ends. He told me they don't plan to make gear sets for at least another 2 years, and maybe not even after 2 years. He said they have to order 150 sets or so to make the price even make sense. And interest just isn't there yet.

I learned more from him in our 20 min phone call than I would have in 2 days searching online. Super nice and informative guy.

As hard as it may be to find a different gear set I'm wondering if it may be time better spent just doing an axle swap. I have no desire to, but I want to be realistic and think about long term use.