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Regarding 6th gear ratio:

You don't really need to physically test the gearing to know how it will perform. You can calculate/model the whole thing extremely accurately. All that is required is accurate Coefficient of drag, Frontal area measurement and torque curve for the engine. It's best to perform a coast-down test to get numbers on the rolling resistance of your specific vehicle, but that's only needed when you are looking for super accurate results.
This makes total sense, although I was hoping to get a far more clear answer by using Brake Specific Fuel Consumption. However obtaining a contour plot would require some dyno time and a fuel flow meter.

Most engines produce the highest efficiencies at low rpm, and high load, generally in between 2000 and 2500 rpm. Do we (the consumers) know this is the sweet spot for the 2J, probably not. Obviously the engineers that were programming the shift schedule of the auto knew this.

(If anyone could provide a contour plot of the stock 2JZ-GE for reference that would be great)

...Then consider that sustaining 80mph requires three times the torque of sustaining 45mph. Ask yourself if you think the engine had 3x more torque than it "needed" when you floored it in 5th @ 2000rpm @ 45mph.
This also can't be more accurate since speed is a measure of velocity squared. 45^2 is just over 3x less than 80^2.

If the answer to this question is a "yes" (which I'd disagree with)
Nope, haha

...ask yourself if it'd be a good idea to put the engine into that speed/load condition in a steady-state situation - for hours on end. Say, 2000rpm at 90+ kPa of MAP. I haven't put a wideband on a stock IS300, but I'd be willing to bet Toyota has it calibrated to start enriching the AFR for MAP over ~80kPa. Engines under high load require a richer mixture to keep combustion, exhaust temps, and catalysts within acceptable temperature limits.
Since the car was built and designed with a W55/A650e, I would also agree. Even though I'm not exactly familiar with the stock tuning parameters. Now if we subjected the stock configuration to this type of abuse, I would be interested on how the stock ECU would manage the setup.


Regarding improved fuel economy

The 30% improvement Tremec mentions is probably aimed at the folks that have a Chevelle, with a 454 big block, with a non-overdrive transmission and a 4.11 rear gear - that currently gets 13mpg. For them, a 30% improvement would take them up to 16 or 17mpg, which is reasonable.
Again, I can see this as well, makes the marketing teams job pretty easy, most likely is their target audience. However even using a generic BSFC countour plot, I wouldn't say it's unreasonable to get 15-20% gains over 25 to the gallon (29-30 MPG hwy). Although this is yet again subject to scrutiny with what the exact engine efficiency parameters of our engine are.


As GGM_IS mentioned, the same power is still required to go a given speed. Air resistance and rolling resistance remain the same, regardless of gearing - so what you save is the friction associated by the engine, trans input shaft and countershaft spinning more slowly in their bearings. This is gonna be worth 1 or 2mpg in our cars.
I did want to touch on this with a separate post, but I can just do that here. Yes it will require the same amount of power, although that power can be achieved in multiple gears. (i.e. 100kW power at the wheels can be found in 2nd, 3rd, 4th, etc., but the lower the gear, the higher the consumption.) It's really just about finding that top island on the BSFC contour plot.

When GGM_IS stated we will need the same amount of power, nothing could be more true. I'm just curious as to what that magical gear ratio would be for optimal efficiency. It's probably just slightly under 0.63:1 with all else being stock.

Regarding graphs

I made those in Excel. I just created a basic table with 7 columns: Vehicle speed, 1st gear, 2nd gear...6th gear. I populated the vehicle speed column with mph figures, from 1mph to 200mph. I populated the gear columns with a basic calculation based on the gear ratio of trans in question, the diff ratio and the tire circumference. Then pulled the table of values into a graph.
And lastly, the graphs. I was able to get some similar results in excel before seeing yours, but you appeared to layout your data with RPM on the Y axis, and speed on the X axis. Mine are reversed when I was generating the tables to get a sense of the different ratios between the transmissions.
 

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So I've just completed my T56 magnum swap (from R154), and put the first ~50-75 miles on it. I have 3.73 rear gears, the close ratio T56, and the McLeod RXT twin...

Wow.

Smooth and quiet, shifts beautifully. I modded the stock tremec shifter and made my own gear stick, which is taller/longer than what most people think is "normal", and also has a dogleg to put the knob closer to the steering wheel. Anyway, the throw isn't super short, but the leverage allows effortless shifting.

The McLeod engages super smooth - huge improvement over a single disk ceramic puck clutch.

First gear (2.66:1) is a little tall with the 3.73 rear gear, so getting rolling from a stop on a hill requires you to know what you're doing - but it's nice to be able to actually drive-through first gear and get up some speed before needing to hit 2nd.

The .63:1 sixth gear is as much as my engine can handle for cruising at 65-70+ (2400rpm @ 80) I can maintain speed just fine, even on uphill grades, but I need to be deep into the throttle for that, and my boost gage reads 2-3psi.

I don't think there's a chance in hell even a turbo 2J is gonna have the fiddlebees to pull an IS300 around with the wide ratio .5:1 sixth gear.
 

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So I've just completed my T56 magnum swap (from R154), and put the first ~50-75 miles on it. I have 3.73 rear gears, the close ratio T56, and the McLeod RXT twin...

First gear (2.66:1) is a little tall with the 3.73 rear gear, so getting rolling from a stop on a hill requires you to know what you're doing - but it's nice to be able to actually drive-through first gear and get up some speed before needing to hit 2nd.

The .63:1 sixth gear is as much as my engine can handle for cruising at 65-70+ (2400rpm @ 80) I can maintain speed just fine, even on uphill grades, but I need to be deep into the throttle for that, and my boost gage reads 2-3psi.

I don't think there's a chance in hell even a turbo 2J is gonna have the fiddlebees to pull an IS300 around with the wide ratio .5:1 sixth gear.
I just put my order in and ill have everything installed in 2-3 weeks. I went for the close since Joel doesn't really sell anything but the close. He set me up with the Clutchmasters FX400 6 puck that I'll be pairing with a lightened flywheel.

Since my first gear in the W55 is shot, i've been starting in second for a few months now. I'm sure 2.66:1 is reasonable compared to 2:1, as I've finally gotten used to it.

I figured if I wanted to push the limits on the speeds to gearing, I can precisely tune it in with different wheel and tire setups. I'm not quite sure how an N/A engine will respond to the gearing since you say you're under boost.

Are you using the Redline MT85 Joel Recommends?

**update: The recommended fluid changed from RL MT85 to Amsoil Torque Drive ATF
 

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Are you using the Redline MT85 Joel Recommends?
I wasn't aware that was his recommendation. That's a pretty big departure from the Dexron Mercon III that Tremec recommends. Wonder what difference it makes?

Back in my mustang days, I had a Tremec 3550 heavy duty 5 speed. I think Tremec recommended Dex/Merc in that one, too. I experimented with a bunch of different fluids. Conventional 10W30, Synthetic 10W30, ATF, GM Syncromesh, the Penzoil knock-off of Synchromesh, 75-90 gear oil... Was never really able to tell any difference, honestly.

MODS: I didn't mean to derail this thread into a T56 thread - but I guess I kinda did. Perhaps you should split this off into a new thread about T56 magnum swaps?
 

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I have a r154 in mine. I used it with the 3.73 gear for 3 years. Decent setup at 575WHp.
This year I changed the rear gear to a 4.10 and its great also dynoed 620whp and I beat the shit out of it drag racing.
Havent had a problem yet. Fingers crossed. Seems like a decent swap to me. If its just the shifter feel that making you spend that extra coin on a different trans... I'd switch another r154 into mine again. But 6 gears is pretty badass.
 
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