Originally Posted by snrusnak
8 speed will be better, but not worth trading IMO. The biggest difference would be starting out power. The 8 speed is geared lower so will pull from a stand still better. The spread of ratios is also better but I don't think that'd be as noticeable. The final ratios are the same, so no difference cruising(other than a little difference in parasitic loss as I'm sure the 8 speed is more efficient).
I have to say sort of. I've driven an 8 speed auto and can attest that the 8 speed is much more willing to let you have that top ratio at moderate speeds. My "6" speed will not readily let me have "6th" gear until 62 - 63 mph in many cases. I've often had to accelerate up to that speed to get my truck into the top gear before letting it cost back down to the 60 mph that I try to keep to in my truck to maximize fuel economy. Once there, it'll hold until 57 mph before a forced downshift will occur.
The 8 speed will happily give you 8th gear @ 55 mph, letting you waft along in MDS mode at 1100 RPM. IIRC, 60 mph was also about 150 RPM lower, so there had to be something different between the two to allow for the slight drop in RPM over my truck.
Yeah, I know I'm a geek for trying to hypermile a truck, but what can I say. I have been able to squeeze as much as 24.7 mpg out of mine, and I think I still have enough room to push it to the 25 mpg mark, as that figure came in despite having to engage in a high speed flat out run to get around a huge piece of oilfield equipment, the pilot vehicles, and a short train of people not wanting to pass, and fighting with the construction zone in Cheyenne WY at the Flying J.