Which is better for my Cobra: Tremec .62 or .82 OD?

[Sharroll Celby]

I have a 351W, alum heads and manifold, lumpy cam, and 3.23 gears, and the T5 with the .64 OD. At 80 in 5th, I am turning 1900 RPM. At 80 in 4th, I am turning 3000 RPM. The Cobra still accelerates in 5th gear from 80 MPH, but at quite a leisurely pace!

I am thinking about a gear change to 3.55s, to get more OOMPH thru the gears, and keeping the .64 OD.

I think the .64 OD would let you get by with a shorter gear ratio and still be able to drive on the freeway, without rattling the fillings out of your head. I have about 30,000 miles with this combo, and 80,000 miles (almost) with the .64 OD.

[RodKnock]

Originally Posted by timsullivan

Rodknock: What size tires are you running with that combination?

If I remember correctly, then I'm running the 275-50-15 Yoko Avids. At some point, I'll be getting the 295's from Avon.

[DAVID GAGNARD]

Originally Posted by timsullivan

I'm building a Superformance with most likely a 351W stroker of around 418 or 420 CI that I'm shooting for 450 to 500HP. I will be running 15 inch BFG's or Goodyear GT2's. My rear end ratio is 3.46:1 I am unable to afford changes to the wheel size or tires and don't want to change the rear end ratio. I will be doing occasional freeway driving here in SoCal and occasional longer trips on the freeway but the vast majority of my driving being simply around town.

Which TKO 600 should I get .62 or .82 OD and why in your judgment? Thank you in advance for any input you can give me on this.

-Tim

Very simple, you know your tire size and rear end gear ratio, you need to "plug in" these numbers into anyone one of the many gear calculators and see what your rpms will be at any given speed......

You also need to know what your engine torque range will be, as stated, don't worry about your peak hp #'s, worry about your engines torque band and try to get your rpms into the torque band..........

I'm running an older Tremec 5 speed in my 65 fastback with the .64 5th gear, 3.25 rear gear and 25.5 inch tall tires, 350hp, 351-W.......I do a lot of highway cruising and a lot of interstate driving in this car... 2000 rpms comes out to right at 75 mph in 5th gear and it's perfect for my motor, I can cruise all day at this level and get 23 mpg........

My cam is a mild camshaft and will run nicely down to about 1500rpms in any gear, so my cruising rpm is perfect for my set-up. If I want to pass someone, I don't have to downshift, just give it more gas and it will accelerate nicely from 2000 rpms.....

With the .64 OD, when shifting from 4th to 5th gear, my rpms drop approx. 900 rpms.......

Your tire/rear gear is known, now you need to determine what rpm level your motor will like to run at and the rpm level your comfortable with at cruising speed........
Just a wild guess, but with a 351-W based stroker, you should have tons of torque and I would think (depending on your camshaft) you could have your cruising rpm level in the 2000 to 2200 rpm range and be fine and still get some decent fuel mileage..........

The only time you'll need 5th gear is on the open road, so don't worry about city driving......

David

[Brian F.]

I have had two T5 transmissions in my roadster. One had approximately a .8 5th gear, the other slightly less, .6 about. With my setup, 351w, 400 hp/tq, 3.55 rear, I like the .6 a lot better. I didn't like the high rmps with the .8. At 75 mph, the .8 was around 2,700 and the .6 is about 2,100 rpm. It doesn't lug, though.

[Got the Bug]

Originally Posted by DAVID GAGNARD

Very simple, you know your tire size and rear end gear ratio, you need to "plug in" these numbers into anyone one of the many gear calculators and see what your rpms will be at any given speed......

You also need to know what your engine torque range will be, as stated, don't worry about your peak hp #'s, worry about your engines torque band and try to get your rpms into the torque band..........

David

Here's a link to the calculator on Second Strike.

http://www.secondstrike.com/Technical/GearCalc.asp

[patrickt]

I still think with the OP's 3.46 rear that the .82 is probably better for him. BUT, if his engine builder says his engine will run smoothly, without hiccupping, at 2000RPM -- forget about accelerating, it doesn't matter -- and 2000RPM translates to a speed that he believes he will be cruising at on his state's highways, then go with the .64 gear. With my 3.54 rear, 2000RPM is 68MPH, and it's almost like coasting with the engine off -- very pleasant.

[DAVID GAGNARD]

Originally Posted by Got the Bug

Here's a link to the calculator on Second Strike.

http://www.secondstrike.com/Technical/GearCalc.asp

That one works great, according to it at 2000rpms I should be doing 74 mph, my speedometer shows right at 75 mph, so it's all correct........

I've used a few of the other calcualtors, but I like this one the best.........

David

[jhv48]

Here in So. Cal. if you are on the freeway and not going 75-80 MPH, you will be rear ended (unless there is rush hour traffic).

My car runs 2200-2400 RPM at those speeds and provides a nice quiet cruise. If I want to accellerate quickly, I downshift. If mild accelleration is needed, fifth is fine. Don't want to be turning 3000 RPM on the freeway. My engine drives just fine at 2200.

To each his own. Do what suits your driving habits, road conditions and engine setup.

[timsullivan]

Jim: I like the sound of that. What rear end ratio, tire size and 5th gear OD are you running? I'm assuming your running a moderate cam as opposed to some high RPM nasty stick?

[jhv48]

I have a Roush 427 SRTW with the dual plane intake for low end torque. My fifth gear is the .64 in my TKO 600 5 speed.
My rear end is the stock 3.46 and the tires are 315X35X17's. My engine purrs at 2000 rpm and hiway cruising is very comfortable at 75-80 MPH (turning 2200-2400 rpm).

Around town in fourth gear at 60 MPH, I'm turning about 2900 RPM. Comfortable for short times and if you need to pass someone, just floor it and be gone.

If your engine is a torker, get the .64. If it's a revver, get the .82.

[WarrenG]

Originally Posted by jwd

You'll find there are 2 types of people. Ones that have the .82 5th gear and those that regret not having it. I have it with a 3.27 rear and it is PERFECT.

Jim

Sorry, three types. I have the 0.64 and don't regret it a bit! Also no problems or issues with shifting or getting stuck in 3rd or any other gear.

[patrickt]

Originally Posted by WarrenG

Sorry, three types.

Jim's brilliant analysis is close, but needs a tweak... there are indeed only two types of people -- those who categorize all people in to two categories, and those who do not.

[RodKnock]

According to Clint Eastwood in The Good, The Bad and The Ugly, there were only two types of people. Those with guns and those who dig. I think the ones with the guns had the 0.82 5th gear.

[jhv48]

Originally Posted by RodKnock

According to Clint Eastwood in The Good, The Bad and The Ugly, there were only two types of people. Those with guns and those who dig. I think the ones with the guns had the 0.82 5th gear.

That means that the ones who dig (.64) get to bury the ones with guns (.82)!

[BeasleyRB]

351W .82 3.25 rearend. 5 great gears.

[Mooch]

I was having the same decision making trouble and made some rpm calculations for the ERA 3.54 and 3.31 rear ends. See pages 2 and 3 of this thread:

ERA #783 - with questions

I've decided to go with the 3.31 rear and the 0.82 5th for the better shift and balance, and I'll deal with the higher rpms and noise at higher speeds - which can be frequent on Florida freeways.

I'll let you know how it turns out.

[mdross1]

I did the rear gear change 3.54 to 3.70,with the .62 od behind a 550hp 460 suggested by Mike Forte.It is the perfect combo for me.Like you have read it is all about getting it right.My first combination was Okay for a while 3.54 rear,with a TopLoader.

[Boostedsnake]

Thank you very much. My plan whas the 0.64. I hate when it rews to much on the highway. But after reading this i have been comfused?

I have 3.54 in the back and driving a kind chevy 350 turbo engine. Do you know how much different its in on the fourth gear in tremec against the munice?
Its perfect in any places but not on the highway.
My idea whas a pure overdrivegear for highways. Not a performencegear.

What do you tryout experts think?

[Slick61]

I've got an aluminum 427W with a fairly snotty cam... aluminum flywheel too. I'm running a 3.73 rear end & the TKO-600 with the 0.82 overdrive. When I first got it, I thought I wanted the 0.64 as well- but as I drove it more, I discovered it was set up perfectly. My engine doesn't come on-cam til about 2400 rpm, so the 0.82 is ideal- especially for freeway cruising. 2700-3000 rpm is an uncomfortable cruising range??? For my engine, it's the sweet spot... sounds amazing without being too loud. My engine could never handle cruising a 1900-2200 rpm... it would buck like crazy. As others have said... it's the combination of engine setup/cam profile and rear end that will dictate the best overdrive for you. Good luck with your build!

[Boostedsnake]

My engine runs very smothly even at very low rpm. And probobly capable to produce around 1000nm+.

Sounds like 0.64 would be good? Or? Its a hard choise!?