I like Ultra-Ramps by Tru-Cut, and their extension. Metal ramps that my 205/60/15 tires fit on perfectly with a few inches to spare on both sides.
have mine for about eight years with no problems.
The few times they have slid, it was never a big issue, just tried a second time and up I went.

http://www.autoramps.com/product_catalog.htm

I bought mine at K-mart, but Griots Garage sells the same ramp