What is the difference between 3.08 and a 3.55 gear ratio in a Class 8 truck?

IF they used the same motor and transmission, the truck with 3.55s would turn higher RPMs at speed, but would be better suited for hills. A truck with 3.55s typically will turn 1500 RPMs at 70 MPH. The truck with 3.08s would turn a lot less RPMs at speed. However, it's extremely unlikely the truck with 3.08s will have the same transmission - it would be geared much too high. It probably has a lower geared transmission than what the truck with 3.55s is using.