Here are my opinions.... others will differ!
** Longest on single axle: 40' overall length
** Wheelbase to length ratio: no less than 55%, (you want a good balance between break-over clearance and departure angle, as well as good driving characteristics)
** Gear ratio: Depends on your weight, driving conditions, transmission, and engine. Best to start with desired cruising RPM, then calculate from there.
Here are some interesting articles on gearing...(keep in mind they're pulling 80k lbs)
https://www.pittsburghpower.com/supp....php?f=7&t=226
10-4 Magazine - For Today's Trucker