Huge difference between an automatic and a 6 speed. The OD gear on a 6 speed is 20% steeper than that of an automatic. First gear is 50% steeper on a 6 speed than an automatic. Certainly there is the multiplication of torque at lower rpms, but in all the gear set is better than 20% steeper.

For this particular application 4.88s is the way to go, and if it were an automatic with the gear sets available, you couldn't get to an equivalent.

 

4.88

 

Getting to 2500 rpm is key, that's where the torque curve flattens out. Beyond that, every additional 300 rpm brings you 10 additional hp, which might be good for moving some additional weight but won't raise your sustainable cruise speeds significantly, perhaps 3mph.

 

I agree, 4.88's. Or 5.13's for some good rock crawling!

 

so .84 is the transmission ratio of 6th gear?

That seems a little high, I had read that 2500 RPM is the soft cap, above which you have diminishing returns on horsepower. So interstate cruising at 75 mph would be 2951.. is this going to kill my gas milage? If I plan on still using this as my daily driver, should I shoot for a 4.56 gear?