what are the best specs on truck? ratio? wheelbase? best mpg

Whoa......

 

Mine was 3.08 rears with a direct 10 12.7DD lp24.5 changed it to a 13 with 3.55 and lp 22.5 if I find a set of 3.70 and than those are going in. Right now running 70 I am 1450 the 3.70 will put me up about 1600 maybe 1550 which is 12.7 cherry spot....I lost mpg going to the 13 with 3.55 and it dropped my rpms from 1700 down to 1450....I was running 7 now 5.5 6 still running the numbers on long term but short terms are not looking good....DD 12.7 HATE low rpms

 

For a highway truck with an ISX, I'd recommend:

Transmission: Eaton-Fuller Super 13 spd (RTLO-16913A) with a final drive (in 8H) of .73
Rears: 3.55
Tires: 11R24.5

That'll give you 73 MPH @ 1500 RPM. If you go with an ISX power plant, they can easily handle 2200+ RPM, but the fuel economy begins to drastically suffer when you get past 1500 RPM, from my experience. So if you're planning to only drive 70 MPH and below, you'll be under that 1500 RPM mark, and realize much better economy.

For highway applications, the Fuller 13spd is arguably the best class-8 transmission ever made.

 

Here's a road-speed calculator that comes in handy. It also reads your parameters and offers different models of transmissions that are suitable.

http://www.roadranger.com/ecm/groups/public/@pub/@eaton/@roadranger/documents/content/ct_062746.swf