Coaxing a 4L60E into Better FE
 

I have a 2004 Chevy Colorado with a 4L60E. Here's what I have found in order to get the TC locked up as early as possible.
- When shifted into 'D' first TC lockup occurs in 4th (top) gear at 80 km/h (50 mph)
- When shifted into '3' first TC lockup occurs in 3rd gear at 60 km/h (37 mph)
- Once the TC has locked it will stay locked as low as 67 km/h in 4th gear (when shifted into 'D') and 52 km/h when shifted into '3'; if speed drops below these thresholds, vehicle must be driven above 60 km/h in '3' or 80 km/h in 'D' in order for the TC to once again lock.
-Under light acceleration: 1-2 shift occurs at 20 km/h, 2-3 at 39 km/h, 3-4 at 58 km/h; once shifted into 4th gear, it will stay in 4th as low as 52 km/h.

What this all means is that if I want to drive at 70 km/h, the TC will never lock on its own unless I first shift into '3', wait a second for TC lockup, and then shift back into 'D'.

It also means that between 52-68 km/h I have a choice between 4th gear TC unlocked and 3rd gear TC locked. With 4th gear TC unlocked, engine RPM is slightly lower, but possibly at the expense of TC inefficiency. Since my ScanGage was stolen, I don't what is more efficient.

The results have been exceptional - FUEL LOG

Can the truck be tuned for earlier shifting and TQ lock? A lot of GM stuff is tunable via HPTuners, EFILive, etc...

Oh probably. But I'm interested in using what I have to its full potential.

What kind of driving do you do? Highway, city, mix? Speed limits?

Mods will differ depending on expected driving conditions.

Easiest for highway would be either larger tires or a gear change.

You can install a manual switch to lock the converter at any speed or gear, it should be in series with the first gear solenoid as not to stall the engine when stopped. Google it, there are tutorial videos on the subject.

I just looked at your fuel log. That's a terrific jump in fuel economy for no cost. I mean - it's a 14% jump over your previous tank, that's huge.

I'm really not interested in mods here - that's not what this thread is about. Besides, the TC can be locked using the methods above at the lowest speed that I would ever want to do it at. I can cruise at 66 km/h (41 mph) at 1200 RPM all locked up.

Don't pay attention to that last tank - that was a highway trip with lots of slower cruising speeds. I probably touched the brake a total of ten times the entire tank.

I just ordered a WIFI OBD2 adapter to run the Torque app for testing. But since I don't have it yet, here's the question:

What is more efficient at 60 km/h (37 mph) - 3rd gear/TC locked/1500 RPM (higher RPM/low drivetrain loss) OR 4th gear/TC unlocked/1400 RPM (lower RPM/more drivetrain loss)? How much efficiency is lost through an unlocked torque converter at low loads?

Determining what's more efficient is a tough call but you'll be able to figure it out once you've hooked up the adapter and fired up the app. Torque Pro has a fuel rate setting; whichever is lowest (set your gear, set your cruise control, then run your test route again in the other gearing but without resetting the cruise control) is your winner.

I am going to take an educated guess and say 3rd/locked is your better bet. It's 1.00/1 so straight through, no or less gearing losses, and no TC losses. I think a lower duty cycle on the injectors will make up for the higher engine speed.