How high a stall converter can I run in a daily-driver 4L60E without killing gas mileage?

Run a lockup converter, and you can go higher on stall without paying for it on the highway.

Why lockup matters: when the converter clutch locks, the converter is effectively direct drive, so slip is almost zero whatever the rated stall speed. That's what keeps cruise rpm and fuel economy normal with a looser converter.
Non-lockup risk: a non-lockup converter can build a lot of heat at cruise, especially in overdrive, where engine rpm drops below the converter's stall speed. On a 4L60 it also requires an extra part installed in the transmission's front pump.
Choosing the stall: base it mainly on the cam's operating range. Lockup lets you lean a bit higher for launch.
Don't overdo it: leaning on lockup to hide a stall that's far too loose overloads the converter clutch when it engages and shortens its life.

The source treats the 700R4, 4L60E and 4L65E as the same for this. Choosing stall with a cam: see the cam finder.

What we recommend

For a daily-driven 4L60E, we'd treat 3,500-3,600 stall as the practical ceiling. In the builds we've seen, a 3,600 costs only 1-2 mpg and is unnoticeable in normal driving, only coming alive at full throttle. If it truly lives in stop-and-go traffic, drop to around 3,200. Fuddle, TCI and Yank are the brands we see used most.

Check this first:

Match the stall to your cam and rear gear
Add a transmission cooler with any stall converter

Based on what we've seen work across our own builds, our friends' builds and the wider LS community, reviewed by our lead tuner (20 years of LS tuning). Every combination is different, so use at your own risk and check our safe tuning limits before any wide-open pass.

How high a stall converter can I run in a daily-driver 4L60E without killing gas mileage?

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