It's no secret I'm a fan of the T56. I've spent some time playing with them.
With any gearbox choice it's not just about the box it's also about the rear end ratio you use with it. The box will have a selection of ratios and these combined with your rear end ratio will give you your speed vs gear and RPM.
Years ago when I was first looking at them I did a lot of math about what the right ratios and looked at lots of different boxes. At the time the TKO 600 wasn't available. The Tremec 5 speed was the TR3550 and it had a pretty ordinary set of ratios. The TKO600 is much better.
I wrote spread sheets and did a bunch of math working things out. I just knocked up a new spread sheet to compare the ratios of the T56 (Holden variety) and the TKO600. I'll do another one to compare the TR3550 as well.
My original calculations lead me to believe the T56 with 3.91 rear gears was ideal.
In the graph below I'll compare a T56 with 3.91 Rear gears to a TKO600 with 3.46 rear gears (a popular ratio in donor cars) and also probably a reasonable choice for the TKO. Rear tyre in my calcs is a 315/35/17 which is about 25.7 Inches in diameter. Traditional 15" tyres will be taller and gear these numbers up a bit.
The graph shows the speed versus RPM of each of the ratios. 1 through 5 for the TKO and 1 through 6 on the T56. TKO traces are in Orange and T56 in Black.
1st Gear and 5th gear are pretty close between the two boxes with 3.91 rear gears in the T56 and 3.46 rear gears in the TKO. Winding out 1st gear will get you to 70KMH, 2nd Gear over 100KMH etc. Note however the spread from 1st to 4th of the T56. They are actually a closer together than the ratios than the TKO. T56 has a taller 1st, 2nd and 3rd gears.
6th gear in the T56 is just a cruising gear. It's only good for loping along on the open road and that's what it's designed for. Note 110HMH freeway speeds tick over at a bit over 1700RPM with 3.91 gears. Good economy etc
Now look at the RPM the TKO is doing in 5th gear at 110KMH with those 3.46 rear gears: approx 2500RPM. A bit High for open road cruising. a 3.08 rear end might be a better choice.
So with the T56 you have the advantage of a close ratio 5 speed with a 6th gear for cruising on the freeway allowing you to run some nice short rear gears.
I'll post up some more graphs if you are interested. I've done them comparing Top loader wide and close ratio boxes too.
If you have a particular box and rear end ratio combination in mind let me know and I can put those numbers in the spread sheet too.
Cheers