taller gear for highway

Hi all,

My 65 GS is running 3200 RPM at 65mph on the highway with it's stock 401/SP300 drivetrain, too high for my liking and for not a fast enough speed. What's my best solution for reducing those revs without compromising too much in off the line fun? Taller gear in the rear, or 3sp AT? For trans swap, is there a 3sp that I can bolt in without modding trans support/driveshaft? Numbers matching car so I am trying to do as little as possible that can't be reversed. Thanks for any and all advice.

Elvin

 

A 65-66 ST400 trans would require moving the crossmember back and shortening the driveshaft.
Any other trans swap would still be longer that the 2 speed trans.
Can you put taller tires on the rear?, That would help a little.

 

final drive ratio for the trans is the same for ST300 and TH350/TH400 = 1:1. you would need an overdrive trans to get less than that. 700R4, 200R4, 4L60E,4L80E, 6L80E .. etc.

You need to know what your rear ratio is now to know what you can change it to - rears have a "range" that the carrier will accept.

Another option is tire diameter - what size tire are you currently running on the rear ?

 

There is the Gear Vendors unit that can be added on the backside of the trans but that would require a shorter driveshaft. You could have a new driveshaft made and store the original one along with the tail shaft housing that would need to be removed to bolt on the unit. If you ever would want to bring it back to original simply remove the new driveshaft and Gear Vendors unit bolt the tail shaft housing back on and install the original driveshaft and its OEM again.

Using a modern O/D trans on a Nailhead engine will require an adapter to bolt the O/D trans to it. If you want better gearing get a ST400 that directly bolts directly to the NH and get a Gear Vendors unit for the ST400. The ST400 will give you much better off the line performance than the ST300. Either way the Gear Vendors unit will reduce your highway RPM by 22% from what it is now. If you were to go with the ST400 and G/V unit then the original trans and driveshaft can be stored if you ever wanted to bring it back to OEM. GL


Derek

 

I am running 26.1" all around, basically stock diameter as far as I can tell. I know final drive would be the same, but was thinking with lower 1st I would be able to run a taller rear end gearing. I suppose if I have to modify the trans support and driveshaft then an OD AT would be the way to go. Would the taller tire gain much, and how would it effect around town?

Elvin

 

I'm turning 2000 rpm at 60 mph with a TH400 switch pitch and a 2.41:1 axle. Nothing short of an axle
ratio change or overdrive unit will get those rpms down. Choose your method.

To maintain performance I put in the TH400 switch pitch. That 4100 lb car manages 0-60
in 7.5 sec with a stock 403, not a hot rod but respectable. I think the 13" converter does slip a bit less for
lower rpm and a point better mileage. I took the 2 speed out of my 68 and swapped in a short tail TH400
without changing the drive shaft, shortest yoke. Same for TH200, TH350. My control always keeps it in
high stall through the gears, helping performance. Bruce Roe

 

Thanks for the advice guys. So it would seem that short of moving up to an OD trans a rear end gearing change is in order. Is a 3.08 gearing a straight swap?

 

You'll need the center section that matches the gears.....
2.93 to 3.23 gears use one center section, and the other center section fits 3.36, 3.55, 3.90 and 4.30 gears.

 

I think first you need to figure out your rear end ratio and type. A 65 would normally have a pretty reasonable rear end ratio for the highway, but if someone has changed the rear then further investigation is needed.

Talking about OD transmissions is lovely, but with your 401 it is a lot of $$ to get an adapter and a good transmission to hook up- then it is way longer than your original transmission. A GV unit, same problems with length and possible tunnel modifications.

Post some pictures and get the rear end ratio- maybe a lower numerical one is available.

 

A rotation of the tire and drive shaft shows just over 3 1/2 turns so looks like 3.55 is my number. I would like to get the rpms down to 2500ish for highway speeds. Will 3.08s kill my in town fun? Looked into adapters for other trannys and you guys were not kidding about the cost, $900ish for most I'm finding plus trans/mount mod/shortened driveline. Is there no bolt on 3sp AT that works on the 401? I would have thought that option would have existed even in 65.

 

There is the turbo 400, aka ST400 3 speed auto, but it's longer than the 2 speed ST300. The crossmember needs to be moved back and a shorter driveshaft is needed. The big Buicks got the st400. the 65-66 Skylarks and GS's could only get the st300. '67 was the first year the GS got the 3 speed auto.
The relocated crossmember will need something to bolt to, so brackets need to be welded or bolted onto the frame.
Will also need to modify the shifter for 3 speeds. Best option is to buy the cable kit from shiftworks.
They say you don't gain any 1/4 mile performance with the 3 speed vs the 2 speed. But the 3 speed is more fun to drive.

The 2 speed isn't a bad trans.... It has the switch pitch converter with a low stall or high stall selected via a 12 volt switch. In stock config, it's connected to your gas pedal/carb linkage. But you can put a manual switch on there to control hi or low stall. (more fun that way!) Bruce (posted above) has some nice controllers that will optimize the performance.

You can put in a converter with a higher stall speed..... one from a v6 will give more stall. That might make up for the different rear gears (if you swap them).

Putting 3.08's into your car will require gears, a new/used center section, and setup.... and stuff like new seals and bearings. Not cheap.
You might be better off just finding a whole rear and swapping it in.
However, the GS has larger wheel bearings than the standard Special/Skylark. The Special Skylark rear will fit, but it's not as heavy duty... but not a huge concern. 2.93 and 3.08 ratios are common for 64-66. Might be worth swapping in a cheap rear to see if you like it.

Some math:
Going from 3.55 rear to 3.08 is a 13.24% change. would reduce your rpm from 3200 to 2776
Going from 26" tall tires to 29" tall is a 11.5% change..... 2832 rpm.
......borrow some 29" tire and go for a drive!

Finally, my '66 GS still has the 4.11 rear I put in a few decades ago. It was happy at 3000 rpm all the way from Pa to Kentucky! But the speed limit is no longer 55, so it's not a good gear for today's highways.

 

For what it's worth, I have a 200-4R in mine with a 4.10 gear and 27.5" tires in the rear. In OD I run about 2,400rpm at 70mph. Couple that with the 2.74 1st gear ratio and it's simply the best of both worlds by not only dropping your highway cruising rpm but also the performance difference in 1st gear that won't believe. Yes, you'll have to move your crossmember back a few inches but you can retain your original driveshaft. You'll need a lockup kit and a throttle conversion for the TV cable but this is all nothing when you consider the benefits of the swap. I'd do it again in a heartbeat.

 

What engine do you have in your car? The OP car has a 401 Nailhead that requires a very expensive adapter in one of the above posts he said one would cost $900 just for the adapter.

If you have a sbb 300/350 or a 400/430/455 then the 200R4 bolts right up behind those engines, the Nailhead it does not.

The OP would need the adapter plus a 200R4 built to handle the torque of the Nailhead so not a cheap swap.

If he found a rebuildable ST400 core for around $150 then just get a stock rebuild for it should be under $1000 and is designed to handle the torque in stock form. Either add a Gear Vendors unit for around $3,000 or if he can find a used one for maybe 1/3 the new price?

Or have a different rear gear installed for highway cruising for around $500 plus or minus depending on what parts are needed installed. This way with the extra gear a 2.48:1 with the help of a high stall to get to the 6.461:1 first gear multiplication ratio of the ST300 with the 3.55:1 rear gear the new ratio can be 2.60:1 to get about the same ratio. The ones that are close to that are the 2.56:1 and the 2.73:1 to chose from that will give close to the same off the line performance as how its setup now.

Of coarse there are a couple others to chose from like the 2.93:1, 3.08:1. 3.23:1 that will reduce the RPM and give extra off the line getup and go from what is there now. If you like how the off the line acceleration is now but would like just a bit more then the 2.93 would be a good choice for more off the line and a reduction of RPM with a ST400.

If you are just fine with the off the line power as is now then stepping down to a 2.56:1 ratio with reduce highway RPM the most out of all the choices without giving up any noticeable off the line performance. The bad part is that you would need to buy a used set of gears, or if you can find one you can buy a complete rear end setup with a 2.56:1 to swap in. A complete rear with that gear is probably the cheapest way to go if you have a place to store the original rear with the original trans and driveshaft if you ever wanted to bring the car back to original all the original pieces will be stored waiting to go back where they came from. In the mean time your engine will last longer because it is now running less RPM in drive than it ever has while giving the almost identical off the line getup and go all without changing tire size. GL



Derek

 

a 200-4r and th350 is the same length isnt it? i know the adapter adds a little length but idk if its enough to worry about. a gear vendors and st400 would cost about the same as a decent 200-4r and the adapter i would think. the 64-66 adapter is only 675 btw.