Must be warm in your sister's pickup, because where I am from the sidewalks are burried in 20 inches of snow. Running on pavement gives me shin splints, not to mention that I cannot control the incline of roads.
I believe Andre Agassi uses one that can go up to 18mph and an incline of 25%! Try doing a google on Agassi and treadmills... It will be pretty expensive though!
I used to train one that whent at least that fast. I think you can try scearching for over-speed treadmills. I forget the company, but i think it was something like wood???...I THINK thats what it started with, but I really cannot remember well.
I just found a company called Rodby online that produces treadmills with a top speed near 15 mph and an elevation of up to 25 degrees. My other option is to look into the treadmills used to train racehorses. They have a max speed of 55 mph and an elevation of 10 degrees