Wow, is it 1952 here in this thread?????
Smudge, I know your young but you obviously don't understand women yet.
If my wife asked to borrow my car, whatever it was, to go see friends/family, I would.
I'd be concerned, but I'd let her do it.
Before she asked you she probably thought she had a great BF, with a nice car, doing well for himself at a young age. She obviously sees your car as something to aspire to and couldn't wait to drive it up and show it (and subsequently you) off to her friends/family. If it all goes well, your well in the good books with her friends, family but most importantly HER.
Now she thinks that your tighter than a mouses ear for quibbling over £40 of petrol, who's paranoid over an 8 year old Nissan which is probably not worth a great deal more than her car. She probably thinks your more in love with the car than her. She also rants to her mates about it who also think your a chump. Now, as a woman, she starts to think about the future ( ); if he's like this now what will he be like when we are married?? Is this the sort of person I want to be with??
My Mrs (she wasn't my wife at the time) begged me to let her take my Gallardo over to a friends to take her out for lunch as a surprise. Did I want to let her? No. Did I yes. Because when she came back from wafting around birmingham with her mate, she was over the moon, as was her mate who still goes on about it now. After a quick check over the car, I was suitably rewarded for my trust