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Old 05-12-2013, 10:47 AM
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MC Fanatic
Join Date: Sep 2012
Boat: 2005 MasterCraft Prostar 197 & 1989 190
Location: North Texas/St. Germain, WI
Posts: 720
Your GMC is under warranty. Take it in and have it checked out. Even with a higher rear differential ratio the lower ratios of the first 4 sets of the 6 speed auto give you a better low end final drive ratio for towing. The sixth gear set is to give the extra miles per gallon rating on highway and all six allows for additional city MPG as under normal driving shifts come quicker reducing rpm. Results help raise the all important CAFE rating. At first glance your issue sounds like a programming issue or a speed sensor.
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