I took the rear axle out of my car (66 F85) originally to clean it up and paint it while I have the trunk pan removed and now I'm thinking I might want to upgrade the gears. It's stamped SA, which my CSM says it's a 2.78. I believe the housing is the Buick style. I plan on keeping the 330 mostly stock, but replacing the 2 barrel with a Quadrajet and proper intake. I might end up going the dual exhaust route at some point, but I wasn't planning to upgrade the cam or anything. Trans is 2 speed jet-a-way. My goal is to have a nice, fun cruiser that looks and sounds good, but I would a nice bonus if it at least felt somewhat quick when I hit the gas. So my question is pretty simple: will upgrading the gears and/or converting to posi give me any added benefit given that I'm going to keep the 330 mostly stock?

 

Installing a numerically higher gear will increase your off-idle performance and a Posi would also be a great stability upgrade from your current setup.

The question as to what ratio you want depends on your use of the car - the higher the ratio, the higher your RPMs climb at speed - With your transmission, 3.08 would probably be a good minor upgrade if you do a lot of higher speed cruising, but if you're mostly a local road cruiser, going up to 3.23 would be an even more noticeable improvement.

As an example, my ragtop currently runs a 3.08 Posi and it runs approximately 3000 RPM at 70 mph. I used to have a '72 Skylark ragtop with 3.23 posi and with similar tire/wheel size combo, it ran close to 4000 RPM at the same 70 mph speed range. They were each backed by stock TH350 transmission. Yours being a 2 speed Jetaway, may be safe to stay conservative.

Others with more Jetaway knowledge will likely offer additional suggestions

 

For the 8.2" ten bolt Type B axle, SA code for 1966 would be 2.78:1 gears. Other available ratios were 3.08, 3.23, 3.55, and 3.90. All were available with the Jetaway. You didn't say what size tires you have, but assuming about 26.5" OD (similar to the OEM 7.75x14 tires that came on the 1966 442) even with 3.55 gears you should only be turning about 3300 RPM at 70 MPH. People seem to freak out over this, but these cars were designed for this, and the speed limits on interstates were 70 at the time.

Sorry, that isn't physically possible. To hit anywhere close to 4000 RPM at 70 MPH with 3.23 gears, you'd have to be running tires that were less than 22 inches OD, or you had insane amounts of slippage in your trans, or your speedo or tach were just grossly inaccurate, or you were in second. If you saw 3000 with 3.08s, going to 3.23s is only a 5% difference in ratio and RPMs with the same tire size. Math is your friend.

 

Thanks!

 

It's a win win stock engine or not. As 70sgeek put so eloquently in his posts opening sentence.

3.23 is a great gear, the 66 A2 442 I had came with it and the 2 speed moved the car swiftly. However i am a believer in, if you are going to do something make it really count when it comes to something like this. And I would go with the 3.55s, Oldsmobile made no mistakes between 1964 and 1972. I would take there suggestions to the bank.

P.S. outside of upgrading to a 3 or 4 speed automatic, swapping to a more aggressive gear is the move. It's the biggest bang for your buck with a stock powertrain.

 

Thank you! I had the 2.78 misnumbered as 2.73.

 

Thanks! After some googling around, it seems the 3.55 kit is more readily available than a 3.23 kit. I guess most people are of the same philosophy: make it really count. haha

 

If it were my car, I would leave the rearend as is and swap the 2 speed transmission for a TH 350 3 speed instead of doing a rear gear swap.

 

Thanks for the suggestion. Why would you go that route?

 

TH 350 has 3 speeds which is an improvement over the original 2 speed transmission that has low and high gears. You get a lower first gear for better acceleration, then a second gear for more acceleration, then third gear which is the same as the “second” gear you presently have.

 

Agreed, the 2 speed is a slug from a stop. With the th350 it will instantly feel as deeper gears were installed in the rear.

 

The biggest bang for the buck would be swapping in a 3 speed trans. The car will drive exactly the same on the highway, but will accelerate like you have an extra 50hp under the hood.

If I remember correctly, the first gear ratio of the Jetaway is 1.82. The first gear ratio of a 350 trans is 2.52. The added 1st gear will make a huge difference in get up and go.

 

Agreed !

Runt as Fun71 laid out. The Jetaway in essence has no first gear. When you multiply the gear ratio vs its "1st" gear it comes out to 4.92. Quick cars are around a 10.0

With a 3 speed you will be 7.01 SLR (starting line ratio) as my 69 is. 66 W30s are 9.53 , and it benefits all cars. G-Body 442s are 10.22 . A G-Body CODE 9 drivetrain on your car with its stock healthy 330, will be as quick as an original 1964 442 and a 400CID 1966 A2 442. That's how significant and important performance setups are. In this case far more important than slight HP mods. With overdrive you can go even more nuts on the aggressive gear side. As Joe P pointed out the 3.90 was doable as was. But with overdrive you won't even notice it till it counts.

Without a doubt !

 

Thanks for the explanations. A trans swap is something that I may do later on in the project, but right now I have the rear out and on jack stands so I feel like it's pertinent to upgrade the gears now if I'm going to do it at all. I'll probably get the metalwork in the trunk sorted out, get the rear back together and do a brake system overhaul and then drive it as is for a bit before I tackle taking the engine out and giving it at the very least a good cleaning and paint job. When I do that, I'll confront the issue of swapping the transmission. But either way, whether I decide to keep the Jetaway or swap to a TH350, a numerically higher gear in the rear will benefit my car, unless there's something I'm overlooking?

 

Though not technically or universally accurate, quick ball park for evaluation is 300 rpm change for each step in gears at 70 mph. Want something more reliable and accurate? Use a gear calculator.

3 speed trans will make a big difference as 2 speed is a dog with arthritis. Acceleration will be nearly night and day. 3.23 - 3.55 will make a big difference too. The two changes combined would be huge!

IMO(bservation) 3.55 is most people’s limit on the highway and beyond many comfort levels. The car will take it, but driver may not tolerate well. Hell, I daily drove cars and commuted weekly Hartford to Providence in cars with 4.11 and 4.56 gears, I survived and so did the cars, but comfortable? Let’s just say I was built of more resilient fiber as a yute!

…

 

To add to what others have posted, changing to the TH350 with it's 2.52 first gear (vs. 1.76 for the Jetaway) means that acceleration in first would be like having 3.98 gears with the Jetaway.

 

If a trans swap is in the future, put 3.73s in the rearend, and swap in a 2004R. Once that is done, the car will accelerate like it has an extra 100hp under the hood, and with the overdrive and the converter locked will cruise at a lower engine speed than it does now.