The consensus among our experts on the subject of limos is: $20.
One of our experts comments, "If that's too much for you, you shouldn't be allowed in a limo in the first place."
Another says, "If you get a comped limo ride, you’ll usually feel cheap if you don’t tip $10-$20 for a ride from the airport to the hotel, or more for longer rides, even if a taxi would have been less expensive."
Our third expert explains, "I always start at $20 and consider going up from there. One time, it was my parents' first visit to Las Vegas and the limo driver took the scenic route, pointing out the hotels and landmarks and giving them a fine history lesson. He dropped them off at one hotel and us at another. I figured it was almost like two rides, so I tipped $40."
As for commercial shuttle drivers, in our view, $2-$3 is perfectly acceptable for a single passenger. If the driver handles your luggage for any reason, $5 is a good tip. If you're in a group, $1 per passenger and $1 per bag are reasonable. The max is usually $5, though if a driver is especially helpful in some way or if you're lugging golf clubs, skis, scuba gear, or other special equipment, another dollar or two will be highly appreciated.