'68 Firebird 8.5 Rear With 4.11 Ratio
 

The 4.11 rear gear in our Firebird doesn't work well with how we want to use the car. Most shows are a few hour drive from our location, and anything above 60 mph is really noisy and nervous. We do intend to run at the local drag strip, Kaukauna, at least once this spring to get a 1/4 mile time with slicks in place. The rear end is quiet and the posi works great.

We've discussed change to a 200r4, overdrive manual transmissions, but the TH400 works great as it is with the 3500 converter.

The engine is a Butler 467 with round port E-heads, Doug's 1 7/8" headers, 3" Magnaflow exhaust system.. We're running a Lunati Voodoo hyd. roller cam, MSD ign., Alum. '66 ported Tripower with modified carbs for more flow.

From what I've researched, 3.23 gears would be ideal for this combo. What do you think?

Here's what it looks like inside.

3.23 is still too much gear. IMO

With that much engine, 3.08 would be plenty of gear, especially with a 3500 converter. But, for those long trips, I'd go even lower, maybe 2.73.

If you really have an 8.5, which it appears to be, any less gear will require a different posi unit. The 2.41 & 2.56 ratios used a different carrier, which is referred to as a "2 series". The 2.73 & numerically higher ratio gears require a "3 series" carrier.

3.23 would be a perfect gear for highway and performance, probably why most muscle cars came with that ratio.

3.23 to 3.42's are a pretty good road to race balance set of gears. 3.55's on the high end. Personally, ~3,200rpm is the upper end I'm "comfortable" driving hours at, though the engine likely doesn't care much beyond slightly more wear. I'm putting the 4.56's back in my Firebird, but have an overdrive transmission and still the engine turns ~3,000rpm on the highway with them, and didn't bother me at all.

Hey guys, he has a Butler 467 with round port heads and a roller cam. Is there any reason why 3.08 gears would not be a plenty of gear for it.

I'd guess that his engine will still produce plenty of tire smoke, even with 2.73 gears.

And, keep in mind, he said he has a 3500 stall converter. My guess is that if it actually stalls that high, it has too much stall for his engine. But, what the heck do I know.

Wheels and tires also factor into this, don't they? It looks like you have 15" Rally II's...

I would work the equation backwards. What MPH do you want to cruise at and at what RPM can you put up with? In my 68 bird I have 3.36 gears with a 25.5" tire 8.2 rear. Its miserable on the interstate (3000 rpm for 65) but its OK at 55 mph. I will be going to 3.08 8.5 rear soon. Not a huge change but Its what in the 8.5 rear and should be some improvement. When I was playing with the calculators the ideal gear I liked for the RPM I was really wanting was the 2.93.

To me it seems the exhaust noise plays into the equation too. The louder it is on the road will dictate how far your willing to drive it before you start saying that trip sounds horrible.

I'm curious with the 4.11 gears, is flooring it in 1st feel the same as revving it up in neutral?

I'd go with a 3.08 gear. If the car isn't mostly strip, then there is no reason for a 4.11. Look how TAman hit high tens with a 461 and OF HR cam with a 3.08. Your motor should be making enough power to move with 3.08s. I plan on using a 3.08 in my '68 Bird with a 494 CI motor.

I agree with 3.23's and 3.08's. But good point made about the displacement of OP's engine, that it should be able to chew the road up just fine with the 3.08's. If it was a 350 or 400, then maybe I'd go 3.23.