I built a set a ramps out of MDF. I took a piece that was 1"thickx5'x6'. I cut it down to 6 1"x5'x1' strips. Then I took 2 of the 5x1 and cut them at 1'. So I would have 2 4'x1' and 2 1'x1'. Then I took 2 more of the 5x1 and cut them at 2'. So then I would have 2 2'x1' and 2 3'x1. I took a 5x1 and put a 4x1 on top ligning them up on 3 edges, and then screwed it down. Did the same thing for the 3x1. Before I put on the 2x1 I cut a 1' long piece of 2x4 and screwed it to the edge of the 2x1 sticking up (the 2" edge on the 2x1). Then screwed the 2x1 on top of the rest. Then I took the 1x1 and butted it up aganist the 2x4, and screwed it down. I also ran screws through the 2x4 into the 1x1. The 2x4 works as a stop the keep from going of the end. When I cut all the edges that the tire would roll up I cut them at an angle to make it easier. This rasies the car 5" over 5'. Basicly you are stacking 5 pieces of 1 inch thick wood each one a foot shorter. I have not had a car yet the could not go up these ramps.
BTW I got the MDF for free. Also, if anyone wants to build a set I will help you out. I can post pics, go in to more detail, and let you know some things I would change (these were the first ones I built).