axle ratio

[rbm]

getting a truck with dd15, 1850 torque, 560 horse. pulling superbs, want opinions on rear end ratios, want good startability. going to have 22.5 rubber, and 18 sp. thinking about3.73, or 3.90

[ralph]

Your 18 speed is a .73 final drive so with 3.73/3.70 rears you will be @ 1,400 RPM @ 100 kph. The 3.90 will put you @ 97 kph @ the same rpm. Will you be running the prairies exclusively or will you be running BC where good startability on the side of a mountain is important? How fast are you going to want to "cruise" at? That really will be the biggest determining factor. You can't get great startability and and great cruising speed all in the same spec so one has to give a bit. What rpm does Detroit want the truck to run @ for maximum efficiency?

[rbm]

on the praries, but worried about starting out in a soft feild, or yard. truck is for farm work. dealer is saying 3.70, alittle worried about the starting out, dont like to ride the cluch too much, kinda spoiled have 4.30 in old truck. dealer is saying detroit likes the rpms down for mpg. im all for good milage, but a little worried about the start up.

[ralph]

And again I repeat myself, it's really tough to get a good startability/highway ratio. Have you looked @ 2 speed rear ends? I had 4:88's and 3:55's in one of my KW's...it would run the highway and get good fuel economy AND have startability second to none!

[Tired Driver]

First if pulling super B off the pavement better have double lockers, with 22.5 rubber the 3:70 ratio would be like running 3:90 or lower. The lockers would help with traction while off the pavement, this would help take the stress off the driveline by allowing the truck to start off with out slipping or riding the clutch to get things rolling in soft surfaces.