Several options are available when going with a hydraulic drive the best option is going with a closed loop hydrostatic type of drive. The people you should talk to are Rexroth or the Parker Hannifin Bent Axis group. With a hydrostatic drive you can have several different control schemes: 1. Fixed displacement pump and fixed displacement motor. This is what I call a hydraulic chain drive. The ratio between the pump and motor displacement is like the ratio of the front sprocket to the rear sprocket on your motorcycle. Speed variation is done by changing the engine speed, 2. Variable displacement pump with a fixed displacement motor or vise versa. This combination will give you a control of the ratio between the pump and motor because the pump volume is variable. Theoretically the ratio should be almost infinite but because of the physical limitations of the pump and motor a good system will give a pretty efficient drive at around 5:1 to 1:1. Most hydrostatic drives are this combination. If you need a wider range of variable speeds then you can go with both a variable displacement pump and a variable displacement motor, this combination will give you almost an infinite amount of ratios.
The closed loop, hydrostatic system requires the least amount of reservoir oil and if you use pressures around 5000 psi and only the very best of motors and pumps, i.e. Rexroth, PH Bent Axis, you may see a system efficiency of around 90%. I you happened to watch the Dakar rally several years ago there was a Yamaha 450 that was two wheel drive and they used components from Rexroth. One more point, as these systems need to operate at high pressures to have good power density they are made from ferrous materials and are not lite weight.
Rex