The 6th gear to my knowledge is not for towing. Also the reason semi's have so many gears is because their's are so close together. Keeps them in their powerband. To add to this they do not spin very fast at all, like ours to a 5.7 or 5.3 gas engine. They spin 5k to 5.5k at redline, something like that, and our Dmax's redline at what 3,100 or 3,200 rpm. Semi's don't have a high rpm so to have any type of speed and power, they need the gears to keep them at the low powermaking rpm's.
Up to 2011 the I don't think their is much differnce in them at all, other then engine and emissions.
2003 2500HD Silverado, ECSB, 3.73, G80, Tekonsha P3, 4" Silverline Dual Exhaust 5" Tips, Autometer Boost & Pyro Guages, Mobil 1 Delvac (TES-295), Opti Lube XPD, 2nd set injectors 109,000, 3rd set injectors 153,000, Cobra 29LTD (peaked & tuned), Wilson 5,000 mileage 172,000
2005 Cavalier 4dr.
1996 1500 Silverado, ECSB, 5.7, 4L60E, 3.73's Powertrax Locker, Lakewood Slappers, mileage 190,000
1974 Corvette, 350, 4 speed, T-tops. Bright red. No tickets yet...
1949 F-1, Flathead 6, 4 speed