Quote:
Originally Posted by snrusnak
LOL at rpin...
mike...your math is not correct. I am very familiar with the ratios of the 45rfe/545rfe/65rfe, the 66rfe, the 68rfe, and the new 8 speed. I don't really know what your point is, I understand the rpm differences at different rpms. The problem is that people think lower rpm = higher mpg, which is only true to a certain point. Try cruising at 600rpm and see how your mpg is with the engine lugging. I have personally verified that I get he same mpg at 70mph in 5th or 6th gear with 3.55's.
And your math is way wrong, BTW. For your calculations to be correct you need to assume a tires size of ~26", which isn't even remotely close to what's on these trucks. At 70mph in 5th gear I am at ~2,000 rpm. At 70mph in 6th ge I am at ~1700rpm. I get the same mpg either way.
Yes 3.21 will be ok, but you won't get better mpg if you don't cruise over 75mph. So why shoot yourself in the foot? Just get 3.55 or 3.92 and get the same mpg(better in the city, actually) and have more power on demand(and less strain on the drivetrain).

It's not my math it's from the site I listed in my post. It doesn't take into account overdrives from either transmission, but you get the point.
You obviously haven't looked at the gear ratios. The 8 speed gear ratios make a 3.21 like the 3.55 with the 6 speed. If you think the truck is going to lug in 8th all the time you're an idiot. 8 speeds give better mileage because you always have the optimum gear for every situation. The 3.55 will most likely make 1st obsolete and possibly make highway MPG less at higher speeds.
I've towed many miles with a 210 HP V6 and terrible gear ratios compared to the 8 speed/3.21. A lot of it with hills. He'll be fine with the 3.21 trust me.