I think you should go with 4.10s. With all my mods i get 16.5-17mpg on the highway. I got 14mpg towing 8500lbs. The gears really helped me with around town driving, and off the line. With as high of a first gear as the 4L80E has, 4.10 will help it off the line. Remember how important the 60' time is in the 1/4.

 

i recomend a 4.10. the 4th gear in the 4L80E isn't the same as a 4L60E. you will notice more rpm in 4th either way. it's .70 in the 60E and .75 in the 80E.