If you let off at 30 m.p.h. you probably will get a hard downshift as the gearing is so low. For instance you take off faster in Low is because of the gear ratio is changed, this in turn causes the engine to slow down faster causing the hard and quick redline. This is normal. If you're accelerating and it down shifts you got a rebuild problem. Low gear is not really designed to go above 15 m.p.h. on an automatic. I do transmission testing (dyno and bench) for GM on the upcoming 2008 Tahoe Hybrid so I've got a lot of time on Automatics.
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