If they don't offer to share in gas expenses, they should be. They are taking unfair advantage of you.
I wouldn't be afraid to ask. And don't do it as your about to go somehwere. Casualy bring it when you and your buddies are having a few beers. Explain to them the costs you run up driving them everywhere and how it would be appreciated if maybe they could help share the costs.
If they refuse ask them how they like walking
