Keep them for car rentals, and use 1k for each rental. Pay rest in cash. They bring down the price of car rental as the taxes and fees are waived as this is a reward and not a sale. Try booking cars on the skymiles marketplace.

example Using 1k miles saves you 30$. But note that the second reservation is non refundable.
Base Rate - Fullsize SUV (USD)
(1) Time & Distance ($505.29/Week) $505.29
(1) Extra - Time & Distance ($84.22/Day) $84.22
Subtotal $589.51

Pay with a Combination of Miles and Money
Look to see the number of miles and dollars needed to redeem for this itinerary.
Move the slider to update the number of miles you want to use. Or enter amount in the miles box. The dollar amount will update automatically.
Proceed with your booking. If you want to adjust your combination of miles and money later, you can do so on the Payment page.
1,000 miles + $557.28