You can generally accrue in a rental car company’s points program (those companies that have one) or earn miles and not both.
Rental car companies generally pass on the taxes for miles awarded with your bill. And most rental car accrual offers are quite weak, such as 50-100 miles per day. In those cases I definitely take the rental car points or credits.
But when there are bonus offers at play, the miles can be the better deal than credit towards free rental days.
Back in the day of course you could earn 1700 bmi miles on a single Hertz rental, and bmi’s points were among the most valuable given their one-way awards with stopover on Star Alliance airlines and cash and points pricing.
Fortunately, Frequent Flyer Bonuses points out that you can still earn a 1000 mile minimum with SAS on 1-day Avis rentals. SAS doesn’t have the most attractive long haul premium cabin award chart for US-based members (and they add fuel surcharges to partner flights) but can be used to redeem quite reasonably for US-domestic travel on United and some other sweet spots.
I don’t find myself renting outside the Avis-Hertz-National triangle often, but it’s worth noting that Alamo offers 1000 American miles on a one-day rental.
On a 3-day Avis rental you can earn 9000 Club Carlson points.
There’s nothing out there that would cause me to rent a car just for the points but there are some reasonable accrual offers when you have to rent one anyway.