Tipping off a Free Ride?

Hey guys,

So I am coming into Vegas in 2 weeks for work and have 1st 2 nights out at M Resort. I have to take Ativan due to flight anxiety so I won't be able to pick up my rental until next morning (arriving in evening). So I have 2 options to get from airport to M Resort when I arrive:

- Ride from a coworker - free but will probably have to pay her back somehow through a favor or spending down time 'hanging out'.
- A promo chip from driving company for a car rental and free shuttle - got it from a family member to use

Just wondering what would be the proper tip for a ride out to the M if the drive is already prepaid? Will help me decide what to do I think...

 

For the limo the casino sends for us, we usually tip $20 each way for the two of us and two checked bags, to go downtown.
M is a shorter, easier ride I would think.

 

When I get a free ride, I usually like to tip the cost of what the ride would be if it wasn't free. But going to M, that would be $30, which is too high. Tip $20. You could get away with $15.

 

Another option would be M Resort's free airport shuttle service:
http://www.themresort.com/blog/2011/05/need-a-ride/

As for tipping on a free ride $10-20.