I'm sure you've already discovered the answer to your question a year and a half ago. But since I found your question by way of having the same problem, I thought I'd leave an answer for anyone else with our confusion.
The size Allen wrench you need to change a Solara tire is. Flathead. Yep. The screws aren't real. There's a notch that you stick a screwdriver into and pry off the cover, revealing the lug nuts. Who knew.
Monday, October 15th, 2012 AT 9:15 AM