it can be used "On top" of a tuner. It doesnt re program the computer like a tuner does, it "Lies" to the PCM. So you could use it on top of a tuner. Im going to bet most tuners would tell you not to. Id be looking for this on a stock application with maybe a cold air and exhaust. I dont think id go throwing this on top of a custom tuned truck with many mods. Just my 2 cents.