Have you all checked into the Poulsen link that Darin put above? They add about 35 pounds per wheel, make something like 7 hp per wheel, are 72 volt, have regen, are fairly portable (attachment bar diffs), and would be easy to install. The company is personally wanting a member of their "dealer network" to install them at present...
The control system is a little simple, and begs some hypermiling mods. I personally would put a microswitch in the switch circuit to key off of the throttle, only coming in when off-idle.
Since I'm not going to be driving my Intrigue like Mario (Andretti), I'll just put in stouter springs to handle the batteries, and rear struts if the old ones are worn out. There's enough built-in variablility to handle the extra wheel weight. (IMHO, anyway...
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