Since a car will roll slightly forward and backwards with the transmission in park, I think I would feel a little safer with a 3x3 cribbing arrangement with the top center member missing, which would make for a larger block as well as a chock in front and behind each tire. You can have space between the members in a 3x3. When I say 3x3 I mean 3 members one way and then 3 on top at 90 degrees. The center intersection would provide a solid column for the tire to sit on. The Army Corps of Engineers recommend overhanging the ends by the minimum dimension, so for a 2x4 or 2x6 they would extend 1-1/2" past the member below it, like a log cabin. This is to minimize the risk of splitting. Probably overkill for a car but thought I would share.