In my experience in the prevailing majority of cases miles earnings on the car rentals are usually not worth the cash price differences to get these miles (except for maybe one-day rentals).

So my personal strategy is to minimize the cash price of your rental (using all the coupons and promotions you can find on the FT and elsewhere or use Name Your Own price scheme if you don't mind bidding) and then just take whatever miles this rental could award you (if any).