Originally Posted by snrusnak
I'd suggest 3.55 or 3.92. I have 3.55 and easily get 20+mpg. I hauled ~1200 lbs in my quad cab from FL to TN to AR and back to FL last july in 110 deg F heat, cruising at 75mph most of the trip(through all the mountains) and got 20mpg avg over the 2600 miles. Best I've done is like 22mpg unloaded at a slower speed on flat ground. Like I said I am switching to 4.10's because I almost never cruise above 70mph and NEVER cruise above 75-80mph and from the experimenting I've done it won't effect my mpg. I just won't be able to get 22mpg at 75mph anymore. But my city mpg will improve.
If I were getting an 8 speed I'd still choose 3.92. At the least 3.55. I wouldn't even consider 3.21, just doesn't make any sense, it's like shooting yourself in the foot.
I realize you probably haven't seen the gear ratios between the two.
65 rfe 1st 3.00 2nd 1.67 3rd 1.50 4th 1.00 5th 0.75 6th 0.67 Reverse 3.00
8sd 1st 4.71 2nd 3.14 3rd 2.10 4th 1.67 5th 1.29 6th 1.00 7th 0.84 8th 0.67 Reverse 3.30
At your 75 MPH figure the eight speed with a 3.21 would turn approximately 2866 RPM. With the 3.55 it would be turning 3150 RPM. Now with the 6 speed a 3.55 would turn approximately the same 3150 RPM. This is all using this calculator. http://www.csgnetwork.com/rpmcalc.html
Now for 1st gear in both at 15 mph the 3.21 8 speed is turning 2875 rpm making towing easier with the 6 cyl and that rear end. The 6 speed with the 3.55 is turning 2025 rpm at 15 mph
I've towed over 2000lbs. often with a v-6 dakota that had much less HP than the Pentastar and a terrible first gear ratio. If he's only towing a couple jetskis every once in a while the 3.21 will be fine. If he got the 3.55 his computer would skip 1st for most driving from what I've heard.