I guess your driven gear would have a rpm of 800 rpm since the no. of teeth are half. So now your rpm is doubled and to keep the transferred power constant the torque would become half of 300 i.e. 150 ftlbs.
I have been trying to design a hydostatic set up using a 16 hp gas engine to power a pump and hydraulic motor for a 1200 pound
urban car top speed of 50 mph effiecently. using a variable displacement pump and fixed motor. no accumalators. I've Got a few figures like engine RPM 2000 Rolling resistance per tire is .127 and speed 0-50mph in about 20 secs [I think] Max grade of incline 15%. But I don't have the Math background to do the more complex calculations such as drawbar, or grade of incline ect...I just don't want to come acroos as a cracked for wanting to do this project.