rear axle gear ratios ?

[35chevcoupe]

Hey all . I'm seeing in my faxon manual that it lists 3.90 as one of the gear ratios , was this a special order option ?

[Brian]

3.36 was standard gear in 3sp and 4sp cars.  3.08 was standard gear for auto cars.  Anything else was special order.  Additionally, 300v8 and 226v6 cars had 3.23 standard with manual cars. 2.78 standard with auto 300 cars, and 3.08 standard with auto 225v6 cars.  Wagons were different--don't have those memorized.

[35chevcoupe]

Thanks Brian . I wasn't sure if this was an option or just a gear option shared with other Gm A bodies . I seen it was an option on the olds 442 site also .
with 3.90s a 425 bored .027 over and dual quads maybe i can get close to that 20 mpg mark . 

[Mark Ascher]

John,

I’m thinking more like 2 mpg.

Mark

[Brian]

This is in the "GS Sizzlers" sales lit book:

[GS66]

What were the choices for 1966?

[WkillGS]

Wonder if the 3.73 was ever installed? Haven't heard much about those.
3.90's are out there,4.30 was an option for 66.
My 66 parts book shows the following:
2.56, 2.78, 2.93, 3.08, 3.23, 3.36, 3.55, 3.90, 4.30

[35chevcoupe]

John,

I’m thinking more like 2 mpg.

Mark

Well hopefully it won't be as bad as the 1968 camaro we rode around in back in the early 80s with 4.56 gears in it .

[option B9]

Hi John, Brian, Mark Walt, Jim  I may have 3.90or lower in my 65 GS convertible with auto transmission, when I'm on the highway going a little more than 60mph my Tach is reading 3,000 Rpm. I could never find any markings on the rear axel tubes due to corrosion ( The GS came from Saratoga Springs NY) It's fast off the line, but NO fun driving on long trips since I really can't go over 60.
  The trip to Flint MI in 2003 took almost Two full days and many tankful's of gas. Before I took my red 65 GS coupe with auto transmission off the road for restoration I had it on the highway many times and it was a pleasure to drive at 60plus Mph, same 65GS but a totally different driving experience. Any input on my axel ratio would be appreciated . 

            Tony
     Frozen in NYC !

[superlark]

In 1965, my Dad worked at a Buick dealership in Rhode Island.  The dealership drag-raced a 4-speed 65 Gran Sport post coupe for a few months before my Dad purchased it (at a considerable discount).  I am pretty sure that it had 3.90 gears in it, but I don't know if it came that way from the factory or the Service Dept changed them before they put it on the strip.  I do remember that it was super fast.

[35chevcoupe]

Tony , 3000 at 60 kinda rings a bell with me . If i remember correctly that was around a 3.73 gear but that's also pretty close to a 3.90 also depending upon the rest tire size .  I know the 3.90s in mine are not correct but i was surprised to see it listed in my faxon manual as an option . It even listed the trans speedo gear as being red .
Either way mine will stat in there for now .

[jmos4]

Hi,

A friend of mine bought a new 65, had 3.73 gears, blew them up sometime in 67/68 as he traded it in on 68 Z28

Tony, you don't say your tire size, but I am running a 3.55 with 245 60 r14 and it runs 3000 @60, 3500 @ 70, 4000 @ 80, sounds like what you might have going on.

One funny thing Pontiac had a 4.33 vs Buick had a 4.30, why?
Also the Pontiac had the stronger 8.2 casting as Oldsmobile would use Pontiac axles for anything over 3.08, 3.08 could have either Buick or Pontiac casting, in the 442's.

Regards,
Jim

[option B9]

 Jim, John I have 14inch 215X70 on all 4 corners. and the rear is a Posi unit. So it could be a 3.73 or 3.90 ??

[WkillGS]

Some useful calculators:
https://tiresize.com/calculator/
.....a 215/70-14 is 25.9" diameter
http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php
.....with 0% converter slip, it calculates to a 3.85 rear gear. 2% converter slip -> 3.78 rear gear. 5% slip -> 3.67 gear

With that info, it could be a 3.73 or 3.90 depending on errors. The 3.73 is a better fit for those inputs tho.

I ran a 4.11 rear in my blue 66 for decades. L60-15's were taller and was also about 3000rpm at 60 mph.

[GS66]

I ran 4.30?s in one of my Dodges for decades but back then the National speed limit was 55. I was over 3000 RPM at about 52 mph. I was running M50-14 tires then. The car now has 3.91 gears and 60 is a good cruising speed.
I have a lot of interest in this topic as we plan to take a long trip in a GS when my wife retires and do not have overdrive. We will take a 66 so maybe the 2.93?s will be fine. I may change out though to 2.78.