[Overdriven]

Let’s start with the 6L80e, unlike the 4L80 and 4L60 which are controlled by the ecu, the 6L80 has its own control unit built in that communicates via CAN bus to the ECU, this requires a compatible ecu regardless of what engine you get. While possible to adapt a newer ecu to an older engine, there’s not likely to be any cost savings but likely more headaches. You’d also be better off getting a complete engine and trans package from the same vehicle to avoid tuning headaches. For example a 2014 engine from a pickup and a 2014 trans from a van (or if the engine and trans are different years) this may require a “segment swap” in the ecu which doesn’t always go smoothly and can complicate the tuning process. Lastly let’s talk about 6l80 mileage vs a 4l60. The 6l80 should get you better mileage around town since it has more gears, can keep the rpm where the engine is most efficient. But cruising on the highway, both trans are going to be in the highest gear, torque converter locked up meaning very little difference if any in mileage. I haven’t looked up ratios for either trans but maybe one trans has a slightly better final ratio for a 1mpg advantage.

Rear end gears. 3.08 gears will lower engine rpm compared to 3.54 gears. But there comes a point where lower isn’t always better. With 3.08 and overdrive, rpm will be so low the trans isn’t likely to shift into its final gear and lockup the converter until you’re going way too fast to be fuel efficient. There are guys with 5.3 4l60 3.73 getting 20-22 mpg highway, 3.54 might increase mileage, it might hurt mileage as you might need too much throttle to maintain speed. It’s a balancing act and it helps if you run some numbers through a gearing calculator, there’s a bunch of them online. Check out the LS swap thread in the “Engine and Trans” section of the forum to see what others run. I always like to look at what the factory does for rear gears as a reference, they pour lots of $ into getting the best fuel efficiency. If mpg is the end all for you though, I hear the 2016 and up v6 Camaro gets 27-28mpg (real world owner reports) on the highway.