Here is my idea for home built wood ramps.

The ramps will be 4" long by 12.5" inches high when finished.

Items needed:
4, 2x8X10" or 2x8X12" (depending on tire width).

Cut lengths:
2 @ 48"
2 @ 40"
2 @ 32"
2 @ 24"
2 @ 16"
2 @ 4" for stops on the last piece

Cut a maximum of 45 degree angle on one end of each piece. Shallower angle of 30 degree if needed.

Start with the longest one and screw them together one at time stacking them. Keeping them a 45 degree angle on the uncut angled edge. The top piece will be 16" long by 10"/12" wide. Screw the 4" piece as a stop on this piece.

Thoughts, if you have 20" or larger rims you may want to scale up by 4" to 6" in length for each piece, to keep more of the center of the tire on each level.