I don't know about this kit... but for about $10K you can convert your car to 100% electric... with that you'll get about 100 miles on a charge, and a charge will cost about $2 in electricity.
just search youtube for "electric car"
Electric motors are quite small (about the size of a watermelon) and can either bolt directly to your drive shaft or to an existing transmission depending on how you want it setup.
The problem with electric motors is the storage of electricity. Most custom electric builds just use an f-ton of batteries... you literally kill 100% of your storage space with batteries. Most of them use up all the extra space in the engine bay for batteries too.
It's a great concept... no oil to change, no cooling system to explode, no gas to fill up, no fumes, no, no engine vibrations, no engine noise (though that's a bad thing for some
) and electric motors can easily make just as much torque as any internal combustion engine... Again the problem is the power storage.
In terms of "energy density" even though combustion engines have really low efficiency in terms of power output for the potential energy stored in the fuel, if you were to hollow out a car battery and fill it with gas it would contain WAY WAY more "energy" than that same battery fully charged.
Most GOOD gas/electric hybrids use special "motors" in the wheels that also serve as the brakes and act like generators when braking...
this helps alleviate the electric energy storage problem to a certain degree since every time you stop you're recycling your momentum and storing it back as electricity (obviously there are losses due to friction in everything but every little bit helps).... the Prius and others doe this which is why they don't need to be plugged in... and it's also why they get better city mileage than highway mileage (on the highway you rarely brake so you rarely have electric energy available to augment the combustion engine)
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Is this guys "kit" snake-oil... I don't know
is it possible that someone could make a kit for ~$7K-$10K that would put motors in the wheels and cut your mileage in half... ABSOLUTELY.
motors cost $2K-$3K a piece, the rest of the cost is batteries and regulators which cost about $100-$200 a piece, and then of course random other small parts to make everything fit together. If someone were to develop a slightly cheaper more purpose built motor (since the motors people use are off the shelf industrial motors) and combine it with a small low cost battery system they could easily price in that range and double your mileage.
check out
http://www.grassrootsev.com/ to see some of the motors and conversions people have done.
I don't know anyone who has built an electric car personally but I do know a few people who have built fully electric motorcycles... search youtube for "quit gas" to see one of them.