Over the past few years I believe car rentals have gotten much more lucrative for earning miles with some ridiculously generous promotions from time to time. Looking at the Q2 2011 Car Rental Bonus Promos Thread it looks like Delta is being a bit stingy at the moment, but there have been some incredible Delta promotions in the past few years with Budget and Avis, you just need to check that thread for each car rental and find the best deal for your length of rental and programs you participate in.