Identified! Look after the jump....
I saw this thing on I-35E in Denton, Texas back in 2009. I saw what looked like an oddly proportioned race car coming up in my mirrors, but couldn't get my cellphone camera ready before it passed me at around 75mph.
It's obviously a 4-wheeler, but it was low to the ground, had very streamlined bodywork including between the front and rear wheels, and very wide low profile tires.
Pretty cool looking, but I don't know how stable is would be on the highway.
Update: I think I've identified it as a GG Quad.
|From Motorcycle USA|