How come you cant get a 3.73 ratio in f350 srw 6.7 4x4 with 18" wheels?

 

The 5R110 has a 1.00 gear ratio in 4th gear, so with 3.73 gears you have a 3.73 gear ratio. The 6R140 has a 1.15 gear ratio in 4th gear, so if you multiply the standard 3.31 ratio by 1.15 you end up with about a 3.80 gear ratio. The optional 3.55 gears give you roughly a 4.08 final drive ratio.