Nope voltage is speed, and current is torque. You need more current, not voltage.
You screwed yourself with a 6:1 speed gear. Your original cart, depending on the model, had roughly 12 to 14:1 differential ratio. You traded speed for torque and acceleration. Put the 12:1 gear back in and you can climb a tree at 14 mph.
You cannot have both torque and speed with your cart motor. You have to pick which one. I have built 3 racing golf carts. My current one does 70 mph and will squeal the tires with a 6:1 ratio speed gear. To do that requires a AC constant torque motor, 650 amp controller operating at 96 volts with Lithium batteries. Hills do not affect acceleration much.
Keep in mind you have a Series DC Wound Motor. It produces maximum Torque at 0 RPM. As RPM increase torque goes down proportionally to Red line max RPM. At max RPM torque = 0. So what you experience with a series DC Wound Motor when cruising along and start to go up hill, you have minimum torque at the higher RPM. So your cart slows down (RPM)and keeps slowing down until the torque is high enough to maintain RPM.
Only way to over come that is with AC motors with Constant Torque at all RPM's. So when you hit that hill, the motor/controller produces more current to produce more torque.
