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

[Jim Vander Wal]

OK, I'll go against the grain.

I have the .64 with 3.27 gears. I like it on the highway as I cruise at 2,200 @ 80mph with 315/35/17 tires.

Where I don't like the .64 is 55-75 cruising backroads, the RPMs are too low and 4th forces the RPM range too high for just cruising.

My use may be different than yours. I run lots of highway (hours/days at a time) and find that 2,200 is at the bottom of my usable RPM range. I also play on the track and find that I can run to 157 in 4th if needed so I don’t shift to 5th on the track.

I've looked at the .82 and run the numbers and just don't like the higher RPMs for long distance highway cruising.

There has been a new post about putting the T56 into a SPF. This really interests me as it offers the best of both worlds.

Jim

[twobjshelbys]

Mine is a 3.54 rear end with a .82 5th. It works very well on the freeway - 2500-3000 RPM at 75-85MPH. The engine starts to lug down at about 2200 RPM at these speeds.

[patrickt]

... with the .64 OD. But, I do like the fact that I redline 5th at 225MPH for those quick trips to the Salt Flats.

[twobjshelbys]

Originally Posted by patrickt

... with the .64 OD. But, I do like the fact that I redline 5th at 225MPH for those quick trips to the Salt Flats.

I never thought of it that way. Hmmm...

Heck I haven't even hit the red line at 4th gear yet. Working.... I did in 3rd though. I think it was about 95 but I wasn't watching speedo only the tach.

[Jim Norris]

Originally Posted by mikeforte

Hi Jim,
I sold a guy a TKO years ago to install in his 428 Shelby Mustang. It had 3.89 gears, 2 4 bbl Holleys & a nice Crower cam. The problem was the cam wanted to go 80-90 mph to run clean and not buck. We installed a much more docile camshaft and he then loved to drive his car at normal speeds. The cam we used lost a tiny at redline and gained total driveability....

Mike,
Thanks for the suggestions. I'll take a look at a cam profile change. I don't relish the thought of cruising at 3000 or more rpm.

[jhv48]

It all boils down to your engine. Will it be comfortable cruising in fifth gear at 2000-2200 RPM?

If not, go with the .82 ratio.

Mine is.

[Ralphy]

I am new to the Cobra World and I am considering the same thing. Looked at the T 56 Magnum. After getting familiar with my car I agree with most posts here. The .62 would be fun. however the .82 will be used more often in my opinion. I am running a 428 plus 03. 4 speed top loader with 3.36 gears. Someone mentioned going with the same gears as the top loader, yes to that also. If you want the .62's go with the T 56 Magnum if you can.

[eschaider]

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

Tim,

You ought to do some searches on here and other places about TKO shifting problems going into third gear. Another good search is TKOs that have locked up in gear forcing you to drive home in whatever gear you parked it in.

Both are real problems and take a lot of the driving fun and turn it into maintenance fun. Driving fun is preferable.

Tremec has a very good transmission in the T56 that does none of the above and if you are after brute strength then check out their new T56 Magnum that they rate at 700 ft/lbs of TQ.

Although you can change once the trans is in, if it is not in yet the T56 might just be the better choice ...

Ed

[Rwillia4]

CA highway crusing is at 80. Go with the T56 and don't worry about it. It cost more but it comes with a bellhousing which will cover the cost and has a great feel.

I got mine for $2100 and love it. I just moved from CA and cruised comfortably in 6th most of the time.

[Mister]

I have the .64 5th and have very mixed emotions about it.

In Western Pa, where we live, it is too long. The speed limits are too low, traffic is too slow and the hills are too steep.

This summer we drove out to Colorado, up through the Rockies and then back home. It was great where traffic was light and the speed limits higher.
The longer 5th is great out on the road but it is too long around town.
Compromises, compromises.................
Mike

[SPF2245]

The .82 will allow you to drive fine on the highway, you don't have to worry about the engine being off cam at lower speeds in 5th, it still usable at the track and in town you can throw it into 5th and enjoy a conversation. The few times I wished I had the taller gear was on long road trips (over 300 miles), but then again, those were the far and few between times the car actually went out. The majority of the time these cars go for short hops, and a usable 5th for fun makes more sense IMO.

[Got the Bug]

Tim,

I'm running 295/50/15 with a .082 Tremec and it's perfect for freeway cruising in CA.

[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.

[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

[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.

[Boostedsnake]

Originally Posted by 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.

You have solved my problem. Now i am going for 0.64, i think?

Are you from Sweden ?

[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?