Being used by my best friends?

okay so lately I've been really bothered by the fact that both of my best friends seem to be using me. I don't think they are maliciously doing this but its extremely irritating and its been going on for so long and now its spiraling out of control. They always want to go places and hang out but no one ever offers gas money even though im ALWAYS the one driving. im always the one they come to to bail them out of something they've gotten themselves into, and neither of them are ever respectful of MY LIFE... which includes my time or cirstances it always seems to be about them! ...they don't do much for me like I got both of them birthday presents... neither got me anything but instead... we all got in MY car and went out to eat where I paid for my own birthday meal and actually had to lend one a few bucks for her food!!! it seems ridiculous. do I blame myself for being so "dependable" or them for always uming ill be at their beck and call? what should I do? its getting extremely irritating and I don't want to end up hating my friends who I've known almost 5 years!