I'm looking at the new Tremec TKX 5 speed and am seeking experienced/educated recommendations on which ratio option might be best for my application.
The vehicle is a 70 Supreme convertible, 27 inch rear tires, 3.42 rear end and a street stout 455 engine. The goal is a fun stop light to stop light ride that we can comfortably run a couple interstate hours to a car show.

The options I'm looking at are below. I'm thinking the close ratio is the best one for me but I'm asking for advice. Any help is appreciated and thanks in advance!

Wide3.27, 1.98, 1.34, 1.00, 0.72

Close
2.87, 1.89, 1.28, 1.00, 0.68

 

With a shorter tire and gear, I would definitely go with the close ratio and longer first gear.

 

To get another angle on answering this question, look at the overall ratio in first for each transmission.

For the wide ratio trans: 3.27 x 3.42 is 11.2:1
For the close ratio trans: 2.87 x 3.42 is 9.8:1

Your 455 has massive low-end torque. The 11.2 ratio is suitable for a drag car with slicks. On a street car, the tires will smoke and you will need to ride the clutch to get traction. No fun for your clutch.

The 9.8 ratio is a little lower than most performance cars use, but most performance cars don't have a torque-rich engine like yours.

Go with the close ratio transmission to make a quick launch less difficult.

It will also give you lower highway cruise rpm.

 

Thank you for the input!

 

I have 500 Hp Chevelle with TK600 .64 5th gear and 4.10 rear.

definitely agree close ratio 1st gear is better BUT make sure you know that the drop from 1.0 to .68 is going to be BIG. Your overdrive will be nice for the long cruise, but when you are cruising at 80 and someone pulls next to you, if you down shift to 4th LOOK OUT......actually DON'T.... you'll have to slow down...…. you will feel like you are missing a gear.

on the other hand, if you go with the .72 5th, the car will stay in the power band nicely and you'll be able to downshift easily... BUT your cruise will be at a much higher RPM and you'll wish you had overdrive!!

SO, just don't underestimate the drop from 4th to 5th and how it will impact your driving habits..

I love my TKO600 and how the car pulls in every gear, but would rather have .72 5th to keep the gears close. If the 6 speed weren't so huge, it would be the best of both worlds for me.

Oh... and talk through how your cam operates at the rpm range associated with your tire size, rear gear, and speed/rpm

 

Great "real world" information. I appreciate the reminder to keep the cam in mind as well. Thanks for sharing your experience and knowledge.

 

In the 5-speed (and 6-speed) first through 4th are the driving gears. Higher gears are for comfortable cruising or speed records.

I have a 4.10 axle as well, and my Olds will turn 115 mph in 4th at redline of 6000. The OP will have similar result. The speed at red line in 4th won't change no matter which trans that the OP chooses.

I can downshift to 3rd below 70 and to 4th below 90, and have a good run left in each gear. The OP will have a similar situation, independent of transmission choice.

The rpm difference at 70 mph in fifth will vary by 6% from the close ratio to the wide ratio. There is a tradeoff as you point out, but it might not be too noticeable. And it won't affect downshifting to 4th, which is always the same ratio.

The biggest variance between the two transmissions is the 14% difference in first gear. Picking which one will come down to, as you say,. cam, axle ratio, and tire size--and how they interact with the first gear alternates.

 

I would also recommend the close ratio gear set. I'm running a Keisler RS600 which has a 2.80 first gear and 0.67 overdrive, with 3.91 rear gears and a decent small block. My combo right now is great with the small block to get the car rolling, but if (when) I do a stroker small block or swap to a big block, I'll also be changing to 3.42 gears.

I drive around town in 1st - 4th, and only use overdrive on the highway or country roads over 50mph. Even local roads where I may be able to drive 50-55mph but with traffic, I tend to keep it in 4th so it's more responsive and doesn't bog. As mentioned the RPM drop from 4th to 5th is kind of significant. This is where a 6-speed is nice, use the first overdrive for mild cruising around town and the second overdrive on the highway.

 

I'm looking at the TKX as well. I was thinking about a 6-speed, but I really, really like the idea of not modify the floor/tunnel to get a modern trans in it.

Mark should be done with my engine soon and I have a similar decision to make. This info helps.

 

Do a build thread, sounds like great combination!

 

https://classicoldsmobile.com/forums...erhaul-153917/