I'm Not Heinous, Am I?

Say I have a friend who's celebrating a birthday today. Say I emailed her to see what she's doing, and she emailed me back and suggested that I go with Butterfly. Butterfly is a great friend who threw me a dinner party at her house for my birthday. However, Butterfly lives 20 minutes out of my way en route to the restaurant, and I'm already looking at a 40 minute drive. It really is not to my advantage to carpool, what with the gas prices and all. How evil would it be if I didn't call her first, and just showed up at party time? It's a little awkward because she got a CC of the birthday girl's email to me, so she's probably expecting me to call. Also, the parking situation could be hell, and besides that, people always want to drink.

Catch 22. If I did one thing, I'd be plumb foolish. If I did the other thing, I'd be a terrible friend, and heinous. What to do? Perhaps the title of this post indicates that I've already made my choice.