I only heard of DealerMiles a couple days ago when I got the new InsideFlyer in the mail and they had an ad in there. You have to look up dealers my state. I found dozens of dealer in California, but noticed that EVERY SINGLE ONE of them was awarding miles for SERVICE ONLY. DealerMiles claims on the front page that you can get miles for car purhcases too, but it seems only a teeny tiny fraction of the dealers they list must (for there to be none in all of California).

I was recently shopping for used car using AutoTrader.com and noticed that A FEW local USED car dealers said "Visa/MC accepted" in their postings. However, the prices were so not in line with private sellers that it wasn't worth it all, even with the mileage I could have gotten from paying by credit card.

So that brings up the issue: In a marketplace so far away from set prices like cars, just because you find someone who will let you earn miles, that's only the start of the battle: Then you have to evaluate if they're boosting their price unreasonably (over other options you have) relative to the value of the miles you get. (The car business is famous for giving you lots of discount options which all work out the same price -- like the old ruse of giving you money for your tradein even if it doesn't run, simply because they're not actually paying for your tradein but simply giving you a predecided discount.)

Btw, one way of looking at the value of car buying from a mileage perspective: Most dealers take credit cards for service, but few for purchases. Therefore you can earn more miles (and probably save money in the process!) by buying out-of-warranty used cars then earning miles on the periodic service compared to buying new cars (where you may not be able to earn miles) and then having a warranty so you don't earn miles on the service either. (And meanwhile you paid 2x, 3x, 4x, 5x as much for the new car compared to a used car. Think how many first class tickets you can BUY for that difference in price?!)