[tdavidl]

so had my engine rebuilt at the beginning of the year to RAIII specs...currently has 3.08 pegleg in the rear, builder will be rebuilding the rear and converting to a posi & he suggested I consider going to 3.55 gears....he says it'll really make a diff from a stop and I won't notice it much cruising at 60mph....

anyone have any thoughts ?

[Dave M]

I have 3.31 gears and do a ton of highway driving. At 3000rpms, I'm around 68mph. Problem is that I like to drive faster. But don't like to have the engine revving any higher than that for long periods of time.


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[OCMDGTO]

All depends on how much highway driving you do and at what speed . Near you 97, 70, & 340 are 65mph and traffic goes 75/80. I am tached up around 3400 at 80 but dont mind for the little I come out there. Where I live you go 65 unless you want a ticket. I would go 3.55

[oldsmobiledave]

Presuming that your diff is an 8.5" in your 71 bird your choices were always 3.08, 3.23 and 3.42. Only recently 3.55 gears became available for the 8.5" diff.

Depending on your tire diameter & how you drive your car the 3.55 may be too much gear. I would choose between the 3.23 & 3.42 gears depending on your trans, converter & rear tire combo.

Good Luck.

ps...if you have a 12 bolt then 3.31 is the ticket.

[bird72]

My build is not for race or town sub 60mph fast, it is for being able to go across Montana if I want, on highways people go 70-85mph on. There is nothing more humbling than an egg car effortlessly going by you at 85 while you (looking vintage cool) are sounding like a drag race revving at 75mph and taching over 3500.

So unless you go modern OD, it is a compromise. Quick with lower mph top end, or top end fast with slower 1/4 mile.

I will be road testing my new nutso setup on highway next spring 2.73 Traction Lock with roller 455HO / T400. I may ultimately go 3.08 or maybe 3.23. Over 150mph top end now capable now, who would take one of these that fast (?) but 90mph is 3K on tach.

I have had 3.55 and 4.11 cars and they are fun..... BUT....... real world dictates fun also. Weekend warrior short run cars are fun deep geared, have a place for sure. Highway non OD cars are mathematically limited. It is a "box" one gets in philosophically. Think hard, as this is one of the biggest choices in car demeanor, road experience, and actually how much you dig driving it.

Wallace Racing and others have online calculator, you can play around Need tire dia.
http://www.wallaceracing.com/gear-speed.php

[Dens71TA]

My '71 Trans Am has the stock 3.42 rear gear and F60-15 tires. Driving at 60 MPH the engine is spinning around 3,000 RPM. On the highway its usually between 3500-4000 RPM. If you drive the car much, you'll be glad you stayed with the 3.08. Traffic moves much faster than it did in 1971.

[Dave M]

I think you can use more gear with stick compared to auto. I'm pretty sure I'm seeing higher RPMs because of my th350


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[tdavidl]

thx for all the replies so far...

if it helps it is an auto...my occasional highway (55MPH balt beltway) trek is a 15 minute ride to work & if I do that 4 times a year that's a lot....otherwise the majority of my use is local, short rides on 40MPH roads with the occasional 50MPH road that I may hit 60-65 on for a VERY short distance just to stomp on it to fully engage all 4 barrels :-)

[oldsmobiledave]

Originally Posted by Dave M

I think you can use more gear with stick compared to auto. I'm pretty sure I'm seeing higher RPMs because of my th350


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Not true. Both the Muncie & the T400/T350 have a 1:1 top gear so cruise RPM is the same.

[Dave M]

Originally Posted by oldsmobiledave

Not true. Both the Muncie & the T400/T350 have a 1:1 top gear so cruise RPM is the same.

Yeah, I got sidetracked and didn't finish my point. What I meant was the torque converter slip will cause your rpms to be a bit higher, compared to a stick car. I've heard it's about 300. Some say as much as 500


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[keith k]

Originally Posted by tdavidl

thx for all the replies so far...

if it helps it is an auto...my occasional highway (55MPH balt beltway) trek is a 15 minute ride to work & if I do that 4 times a year that's a lot....otherwise the majority of my use is local, short rides on 40MPH roads with the occasional 50MPH road that I may hit 60-65 on for a VERY short distance just to stomp on it to fully engage all 4 barrels :-)

Given that usage, you'd likely be fine with 3.42s or 3.55s.

But you will unquestionably notice the difference vs. your 3.08s -- both in the acceleration and in the highway RPM. No free lunch.

[OVERULD]

My 74 Formula 455 had 3.73 gears with a posi carrier in it when I bought it. I changed it to a 3.08 to match what it left Norwood with, and I'm very happy with the 20% reduction in RPMs. It still will break the tires loose and lay rubber for about 100' at WOT and either bark them or just keep spinning them as it hits second. Much nicer on interstate cruise at 65-70 MPH. I think my 71 455 H.O. GTO with AC, 4-speed and 12-Bolt, had either 3.23 or 3.31 gears. That was a great gear for that car! Drove it from Atlanta to Dayton, OH, and back for the 2009 Co-Vention and it was a fantastic cruising gear, but would just anililate the tires at WOT in 1st and 2nd.

BJ

[Formulabruce]

Automatics have a Parasitic Loss to the Driveline Vs a Std Transmission. This is why Sticks are usually rated as getting better MPG. So the automatic will increse the RPM for a given speed constant, vs a Standard with both having a 1:1 Transmission output. A close Exception in a 3 speed auto might be a TH350-C with a "Lock up" ( introduced in 1981 in Pontiacs, maybe Firebirds)

[poncho-mike]

I dismantled a fairly low mileage 81 Turbo T/A for the 3.08 posi and four wheel disc. I saved the tranny because I thought it might have a locking torque converter. I never found out for sure if that tranny locked up or not.

I prefer the 3.08 and 3.23 gears with any automatic. I thought the 3.42 had me turning too many RPM at interstate speeds.

Originally Posted by Formulabruce

Automatics have a Parasitic Loss to the Driveline Vs a Std Transmission. This is why Sticks are usually rated as getting better MPG. So the automatic will increse the RPM for a given speed constant, vs a Standard with both having a 1:1 Transmission output. A close Exception in a 3 speed auto might be a TH350-C with a "Lock up" ( introduced in 1981 in Pontiacs, maybe Firebirds)

[tdavidl]

thx for the advice folks, i'm sticking with the 3.08 and will get the posi installed in two weeks....