There usually isn't. There are ways to use miles for rental cars, but they're typically a poor value. And watch out for redemptions that don't include mandatory extra fees and taxes and options!

I use miles for airlines flights and points for hotels stays left and right, but I've never found a good value way to pay for longer car rentals. (Sometimes you can get a "free day" certificate, but these generally are not possible to combine for a whole week, and again they're notorious for making only a tiny part free and then requiring you to pay for a bunch of stuff the certificate didn't include.)

If you haven't booked your flight yet, however, you may want to look into booking a flight+car "vacation" from the airline; sometimes that gives very big discounts on the car (compared to pricing it separately), but not always, but still it might be worth a look. (I've never done this for Europe. The times I've done it within the US, all optional coverages were included in that "vacation" rate, so I saved more than I thought I would.)