The 2 x 8s are by far one of the safest, most stable set of ramps you can build for yourself. 25 or so years ago, when I was strictly a DIYer, I had a problem getting a car onto steel ramps. After some thought, I used two 2 x 8s and had nice ramps that when assembled three layers high, gave me an additional 4.87" of clearance. Enough for most oil changes and small stuff. With today's wider tires, 2 x 10s might be even better, so much better that I built a set of ramps four layers high w/ 2 x 10s. Years later these really helped with a very low car(a 2005 Corvette on a Sunday afternoon- oil & trans fluid) even though I don't really need them often. I had to use an additional board on each ramp to lengthen the accent distance to avoid scratching the front fascia of the Corvette. These are also wonderful to safely back the car up onto for differential service (just be sure to raise front of car w/ jackstands or boards to make the differential level when filling).
Dr. Olds