You might try the waterwheel factory at
http://www.waterwheelfactory.com.
I did some rough number crunching based on your 6 feet of head and 100gpm. I came up with about 60 watts. Utilizing the waterwheel factory calculations, I came up with 96 watts on the shaft. However, the fine print will tell you to reduce that number by 1/3 to � to allow for gearing and battery losses, which would yield somewhere between 48 and 64 watts.
If your car wash design includes shutting down the water flow for breaks in customer flow (to save energy), the repeated stopping and starting of the water flow will severely impact the total quantity of power produced.
You would probably be better off to put whatever money you have set aside for the water wheel into energy conservation measures. If you have already conserved energy to the extent possible in your design, and still want to incorporate generation, I would suggest looking at putting solar on the roof.