Going from 2:73 open to 3:23 POSI on a 66 'Lark - what's will my HWY RPM be?

Like the topic says.

Needed to have my rear axle bearings replaced and I've been thinking about going to a POSI. Shop guy tells me I can get a POSI carrier for 2:90 - 3:23. Problem is new gears are *expensive* but he thinks he can get me a good used set of 3:23 pretty resonably.

Problem is I use this car daily - I don't have to drive on the highway very much but I like crusing along at 65 doing 2200 RPM.

How is the move to 3:23 going to change my RPM on the Highway? How will it change my acceleration?

Car has a freshly rebuilt ST3000 with the switch-pitch. Motor is a '66 300 converted to a 4bbl with new cam, pertronix, and dual exhausts.

Love to hear the PRO's/CON's of doing this swap - from open to POSI. All opinions welcome.

Thanks!

 

Well your going to love the new found acceleration :3gears:

 

With a 28" tire you will be right around 2500 rpms.... I run that same gear with a tight 10" stall with a 28" tire and i run about 2600 rpm's at 65 or so....

 

I like the 3.23 ratio very well. I went from open 3.23 to posi using same gears and it made a great difference for the 455 off the line. Tires are 245 50 15 (if memory serves) and rpm @ highway speeds is bearable @ less than 2800. my stg 1 has 3.64 gears with the little 225-70-14's. Highway rpm is over 3000 rpm and it kinda sucks.
I did not think the 3.23 : 1 gears were too easy to find!

 

if u go to the 3.23 and make no other changes, u will tach out at ~~ 2600 rpm for 65 mph.

 

Here's a great calculator page. You can play around with tires, gear ratios, rpm's, transmissions all day long.

http://www.1bad69.com/calc.htm

 

Thanks guys. We'll see what the guy at the shop comes up with. Sounds like a good upgrade.

 

As mentioned above, 3:23 was proving hard to find. He did find a set of mint, 66 3.08 gears off a Buick however for about $100.

I was sort of looking forward to the 3:23, but not I like the compromise. 3.08 is still an 11% improvement in ratio over the 2.79, vs. a 16% improvement for 3.23, but it will only raise hwy RPM about 250 @ 65.

Whats interesting is that one person above is running a 28in tire vs. 245-50r15 which is only a 24.6in tire. That's a big difference! I'm running 245-45r17 which is a 25.6 in tire, and I think that's the same overall diameter as stock. The difference between 24.8 and 28in tire is almost 300RPM at 65!

 

Great calculator! Especially that it provides the gearings for many popular transmissions.

Answered my question as to whether a 3.55 or 3.90 would be best with my 200-4R. Looks like I could use the 3.90 and still have lower rpms at 80 mph than the original drivetrain. Thanks for the link.