[chuckies76ta]

Mick: I run 3.73 and a TKO-600 with .82 od. My engine combo doesn't like cruising below 2200 rpm. The .64 is much better if you can drive at 80 mph on the freeways. Here our speeds are limited to 60 to 70 mph.
I can cruise at 65 to 70 mph no problem. my rear tire is 28" tall. I believe I cruise at 2500 at 70 mph

Let us know how the rear gear and od work out for you once installed.

[Formulajones]

Originally Posted by PDC

I just got tired of the significant drop between 4th and 5th with the .64 OD. Anything between 70 and 80 felt like ‘no mans land’ where 4th was spinning faster than I cared for and .64 od felt soft. Not trying to start an argument here. Just saying what worked for me. P.S. - How big is the drop between gears with the ST-10 If 1st Gear is 3.42? The gear spread with the TKO 6 is quite a bit more functional and a 2.64 final drive in 5th is still a bit calmer than 1:1 with a 2.73 out back.

The difference is only 3 mph at the same rpm. So you're essentially doing the same thing.

[Formulajones]

Originally Posted by tom s

With the TQ of our engines I just have not had a issue with the drop from 4th to 5th.Tom

If I'm not having an issue with a little 302, then these big Pontiacs should soak it up no problem.

With my .64 overdrive and 3.55's out back I can cruise all day long at 65 mph and 1800 rpm, and still climb all the mountain grades around here without downshifting. I don't have to cruise 80 if I don't want to. And that's with the smallest production engine that's likely on this forum, lol.

All the swaps I've done and I've never experienced the .64 issue some of you guys are talking about. It sounds like maybe a tuning issue or a combination of engine parts that aren't very happy together.

[Formula8]

3.73 and TKO600 here. With my 455 and SFT cam, for a street car obviously first gear was a lot but I rarely do hard clutch drops from a "dig" anyway and it was a perfect combination in my view as I could lug down in 5th without an issue. That combo would get 11.5-12" of vacuum and idle pretty easily at 750 rpm.

Now with the 433 and roller cam, it has not been as happy with the 4th to 5th thing as PDC describes. I have some tuning issues to work through now that I am at 10.5-11" of vacuum. I think the springs in my distributor are too light for 1500 RPM and the timing might be bouncing around which may be contributing but over the winter, I am converting the car to FI and timing control so we'll see.

Either way the 3.73 combination is going to be pretty hard to beat and I don't think I would go with much less than that.

[Formulajones]

I'm betting the fuel injection and timing control change will cure all your woes with that deal. Most of it is self tuning and precise control will make things happy in a much shorter time.

Takes quite a long time of driving and trial/error to dial in a carb and distributor as well as a vacuum advance, and find what makes the engine happy in all scenarios when you're trying to lug an engine around in overdrive that has a rather large overlap camshaft. Especially a small cube engine. Eventually though it can be done.

Since you're planning the swap, it's not worth the time and effort to fiddle with what you have now.

[ta-kid]

I run 3:73 with my TKO 600 and yes,1st gear is short shifted or skipped altogether on the street,but doing 2000 rpm at freeway speeds is a pleasure.I run 255 60 T/A,s in the rearSo a 27 inch tire and may go to a 235-70 in the future a 28 inch tall tire to help with the aggressive first gear ratio.

[lust4speed]

Ordered the Dana 60 with the 3.73 gears this morning. About a four week wait and that puts us right at New Years, so maybe or maybe not delivered that week. Trans will be here way before then so should have that in and be ready to install the rear when it arrives.

Back about a dozen years ago when I was taking the car off the dragstrip and putting it back to street duty I changed out the T400 trans for the Muncie but left the 4.11's in for about a month. It was a blast on the street compared to the 3.36's that eventually went back in. I just stayed off the freeway for the month. The final drive ratio with 4.11's and the 2.52 Muncie first was 10.36:1 and the TKO600 2.87:1 first and 3.73's is a little lower at 10.70:1. Bad enough waiting until Christmas for something, but waiting until after New Years is terrible.

[Formulajones]

I guarantee you'll be totally thrilled with that combination Mick. I've never had a customer tell me they hated it.

That's actually right about where I like my first gear multiplication, no matter what engine is in front of it. I like the ease it puts on the clutch in stop and go driving, and hey, if the car doesn't hook up when you get after it, then it just needs more suspension tweaking and a better set of tires

[dld]

this is a good tool.

I went with a 3.55

5800 rpm 26 inch tire is 126 mph . thinking 1/4 mile not wanting to make a shift near the end

http://www.tremec.com/calculadora.php

[lust4speed]

Or you could remove one or both of the other limitations -- the 26" tire and/or the imposed 5,800 RPM limit.

One of the quirks of the math is that going from a stock sized tire like the 26" to the 28" just happens to be exactly one click on the rear end ratio. So the math pretty much works out pretty close to the same for a 26" tire and 3.55 rear to a 28" and 3.73 rear. The TCI calculator does show 129 for my 3.73 and 28" tire, but close.

[Formulajones]

I run a 27" with my 3.55's and it works out pretty well on the street in all conditions .

Engine likes 7,000 rpm though so I have a complete 4.88 rear I use with a 28" tire if I want optimum times. Funny even with that rear it still cruises 65 mph at 2500 rpm and 70 mph at 2700 rpm. Still better than the M-20/3.55 setup that was 2800 just to go 65 mph.

As it sits right now with 3.55's and the TKO it easily crosses the finish line in 3rd gear, lol.

[dld]

well the car in working on is a 70t/a 495 and a fairly small HR cam 336/342/116. so thinking RPM's 6000 max.

The 17 18 inch wheels the tire choice is 25 26.5 . really dont have much option for taller tire.

also this motor should handle the taller gear and turn it into more acceleration

[428goat]

I went with 3.55. Works for me

[TAtech]

I agree with the .64 od gear being useless unless running faster than 80 mph. I built a large displacement aluminum hemi, 800 hp large cam for a 68 Doge Dart several years ago and used the TKO600 with the .64 od using a 373 diff. gear and hated it. I was going to change gear to 488 but car was sold before I got around to it. I personally would not consider anything higher than s 410 with a .64 od.

[TAtech]

Originally Posted by TAtech

I agree with the .64 od gear being useless unless running faster than 80 mph. I built a large displacement aluminum hemi, 800 hp large cam for a 68 Doge Dart several years ago and used the TKO600 with the .64 od using a 373 diff. gear and hated it. I was going to change gear to 488 but car was sold before I got around to it. I personally would not consider anything higher than s 410 with a .64 od.

Forgot to mention it was s 28" tire

[LeighP]

My 68 Firebird with 428 and RAIV cam runs a TKO-600 with a 3.5 rear gear. Overall, the gearing is perfect for the first 4 gears. 5th requires me to be doing 50mph minimum to use. On a highway at 70mph, its terrific.

[Formulajones]

In every case I've seen like that, tuning cures it.

With large overlap camshafts, the idle to main circuit transition is critical, along with power valve all has to be custom modified for smooth transitions so the engine isn't bucking and complaining at low rpm speeds. This is where you toss the wide band out the window and start giving the engine what it wants.

The distributor is also key, primarily the vacuum advance mixed in with the initial timing and low rpm curve to give the engine the right amount of timing to be happy in those conditions.

TAtech didn't give enough information but in your case Leigh, a 428 with a RAIV cam would be really easy and should practically idle along in 5th gear once everything is happy together. 230 @ .050 on a 113 or 115 LSA like a RAIV cam is, in a 428, is a pretty mild camshaft, providing it's installed around 108 ICL and not retarded.

[David Jones]

My TKO600 has the .64 od. I'd factor in the cam for sure. If you've got a LOT of cam, I'd go 3.90 or more. I started out with a 3.55 and decided pretty quickly that a 3.73 would be about perfect (230/236 on a 110 in a 455). It is with my set-up. I find the engine is pretty happy down to around 13-1500 rpm in 5th (45ish)....if the conditions are right. There is NOTHING like motoring along on the freeway....and keeping up....at 2000rpm or just a bit more. As noted, tuning both the carb and the dist are important. If not done correctly you'll enjoy a bucking mess at those low rpms.