Here's a reply from one of my e-mail friends:
Ben
If you want to drive cross country at 65mph with no worries then Tom B's 15KW generator sizing will give you the "serial hybrid for $4500" smirk. If you can't afford a big power plant then you cannot go as fast for as long. A nice tool to give theoretical power required at the speeds you plan to travel is here:
Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com
So for rough figuring at 35 mph your metro requires about 2700 watts of motor output (the spreadsheet includes drivetrain losses from motor to the road).
Unfortunately in a serial hybrid all of the inefficiencies in the energy chain "de-rate" this answer. As Tom G stated if you can eliminate the battery charger that is one less loss( or at least a smaller loss) to deal with. If your generator output is for 120 volt service normally you could size your battery pack for say 108 volts and directly rectify to the battery pack. Others with electronics experience may have other solutions as I was born 50 year too late. My guess is your 3700 watt will allow you to drive at around 35 mph in an urban cycle. Stop lights and slower operation will allow your generator to catch up with other inefficiencies such as hills etc. The key is to not drive faster than what your generator can output over time. (The batteries would only be supplying energy for acceleration for example). Carefully monitoring the amperage and voltage output and temperature of the generator while your driving is required unless you employ a solution that such as Tom B's generator spspigotTBSGS).
As far as setting the generator to run at full speed, I would let it operate as designed. Chances are it will be running at full speed most of the time anyway . If he pack is charged and your at a stoplight there is no advantage to running the generator at full speed.
That's pretty much exactly what I am looking for. Range extention for driving more or less city speeds. Some sort of basic homebuilt charger that would do a set voltage out from the generator and control amperage sounds like the right thing.