I was just kicking this idea around, and it seemed to me that 18 inches for each riser was too long. You could tighten that up, increasing the angle of the rise while still not scraping your car on the ramp.

I would like to get 12 inches of height out of my ramps. Since 2 X 10's are actually 1.5 inches thick (approx), you need eight of them. To get an angle of about 10 or 11 degrees for the ramp, each board needs to be 8 inches longer than the one above it.

Top board: 24" long (2'-0")
Second: 32" (2'-8")
Third: 40" (3'-4")
Fourth: 48" (4'-0")
Fifth: 56" (4'-8")
Sixth: 64" (5'-4")
Seventh: 72" (6'-0")
Eighth: 80" (6'-8")

That makes the ramps almost seven feet long, but a 12" rise on them would make working under there much easier.