You have the right idea I think.
Just adjusting PHY SET VEHICLE MOTOR TORQUE should give you the impression of gear changes, and changing PHY SET VEHICLE MAX MOTOR, or capping PHY SET VEHICLE MOTOR FORCE each gear change should work to control the max speed of each gear.
I've owned DarkPhysics now for several years and am just now working on something that uses the vehicle physics. It's certainly been a challenge getting it working with a model that wasn't designed for it from the start, but I'm getting closer to something that's drivable without bouncing around like a kid in a bouncy house.
