[PDC]

The 2.87 First Gear in the TKO 600 is pretty aggressive, when you consider 2.21 is 1st in a ‘close ratio’ Muncie and 2.48 / 2.52 is the 1st gear ion a TH400 / 350 respectively. I know lots of folks end up going with 3.73s behind their .64 OD TKO-600 in order to make OD ‘useful’ at less than 80 mph. But with the 3.73 rear end, your overall reduction in 1st gear with 3.73s is 10.7. That would be like running a 4.33 behind your TH400. That’s pretty darned aggressive in a big displacement, long stroke engine (455 or 460 stroker) that already makes TONS of torque. 1st gear almost becomes a waste - it’s pure wheel spin for a split second before you have to shift.

I had a .64 OD TKO 600 in front of a 3.42 ring gear and I found that I was never happy on the highway. Even though the engine may not have been technically lugging it just felt ‘soft’ at any speeds under 70-75 in OD. I really needed to be going 80+ to feel comfortable in 5th. And then if I did downshift to 4th from that .64 OD the tach would LEAP to match revs. At speeds between 65-75, I felt like I was either whizzing like a sewing machine in 4th or near lugging in 5th.

I ended up re-building the TKO with a .82 main shaft and swapped in a 3.23 ring gear, and I could not possibly be happier. Overall gear reduction in 1st gear is 9.27. That’s even a bit more aggressive than running 4.11s behind the 2.21 first gear in the stock ‘close ratio’ Muncie. In 5th my overall reduction is 2.64 which is plenty comfortable at 75 mph. And if I need to downshift to pass, its a much more comfortable step. I think the TKO-600 with the .82 OD is tailor made for a 3.23 rear end.

[Formulajones]

Originally Posted by PDC

I think the TKO-600 with the .82 OD is tailor made for a 3.23 rear end.

Only problem with that is you're really cutting yourself short on the overdrive idea. You're still turning 2500-2600 rpm to go 75 mph. Don't need an overdrive to do that.

May as well save your self a bunch of money and stick a ST-10 in the car with a 3.42 first gear ratio, stick a 2.73 rear out back, and do essentially the same thing for a whole lot less.

[Formulajones]

To take that even a step further. We ran a couple of cars here with the T56 that has a .50 overdrive 6th gear. Used that with 3.42's out back in 2 different cars and it worked beautifully. Would cruise under 2,000 rpm and keep up with highway traffic.
I eventually changed one of them to 3.73's. Mainly to improve the first gear multiplication with it's 2.66 first gear. Still went to sleep in 6th and knocked down 26-27 mpg highway.

Did 3 TKO swaps for the same customer, all of them .64 overdrives. 2 cars had 3.70's and one had a 3.50 gear. Driving them really wasn't much difference between them. Hardly noticeable really. The gear change really isn't enough to see more than 100 rpm on the tach and not enough to affect mpg either. After several months of driving the owner had no preference one way or the other.

[PDC]

Originally Posted by Formulajones

Only problem with that is you're really cutting yourself short on the overdrive idea. You're still turning 2500-2600 rpm to go 75 mph. Don't need an overdrive to do that.

May as well save your self a bunch of money and stick a ST-10 in the car with a 3.42 first gear ratio, stick a 2.73 rear out back, and do essentially the same thing for a whole lot less.

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.

[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.

[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.