Overdrive an Overdrive

[red_dirt_okie]

I have a toyota 5 speed manual tranny and axles with 3.73 gearing. Does anyone have experience with a ranger torque splitter and this set up. I was wondering if I could get good gas mileage by overdriving the overdrive. Though I haven't done the math I would guess it would tack at about 1800 rpm running 70 on the highway. The v8 would be lugging but considering an engine is just a big air pump, less rpms means less air/gas used. I know the 2f I have wouldn't pull this but maybe a big v8 would. On the other hand I am not sure a 6.0 vortec produces any more hp or torque at 1800 rpms then does a 2f. Anyone want to chime in?

 

[CruisinFJ60]

1800 may be a bit low for it to really give a fuel increase of signifigant diferance. since you would be lugging the engine some that is added load meaning more fuel to push through the load.
if the engine is set up to have its torque and power in that rpm you may see a slightly higher increase but in my opinion you would just be better off running an NV4500 which has a longer overdrive in stock form as opposed to the H55F and will still keep you in that sweet spot.

 

[rover67]

i think it'd be too low... even with the 6.0.

that's just my opinion tho... do some reading, figure out where your motor is most efficient... i know my 5.3 really does not do well cruising at below 2k at speed.

 

[ntsqd]

Engine's are their most efficient when operated at their peak torque RPM. The term used is "BSFC" or "Brake Specific Fuel Consumption." What that means is that this is the fuel used per HP produced. The lowest BSFC always happens at the Torque peak. So you want to gear the vehicle to put the engine on or a little under it's torque peak rpm when going the cruise speed that you want.

I use OD's. I do not like them. They do not make sense.
Think about what exactly an OD really is. Another set of gears in mesh, turning on additional bearings. There is no way that such a system is more efficient than a straight through coupling. By design an OD has more parasitic loss than does a straight through coupling.
AFAIC transmissions should have a lower first gear combined with a direct high gear. No overdrives. You would end up with the same spread (range) of trans ratios, just all of them moved down to the point that the highest was a direct coupling rather than an OD. Then put a final drive ratio in the vehicle that operates the engine at it's ideal rpm for the intended cruise speed. This also results in lower numeric ratio ring & pinion, which is stronger. If the trans had a lower first gear, then you could run, say, 3.70's instead of 4.56's and still have the crawl ratio that you need while not needing an OD and it's included inefficiencies.

 

[lehiguy]

I gained a substantial increase in milage by switching to an NV4500 and an NP241 T-case. The additional 10% overdrive no doubt helped, but I think it was largely due to the T-case. The 'Cruiser T-case wasn't designed for efficiency; there must be a 20 to 30 HP loss at freeway speeds turning the gears in a thick soup of gear lube. The NP 241 is turning very little extra stuff when in 2-high and it's doing it in ATF, not gear lube. Anyway, I love the setup!