Here's a riddle that came to me in a dream about a car club last night:

The more you use this, the more you have. What is it?

The answers will be screened until someone finds the answer I had in mind.
(I'll be interested to see if someone comes up with a different answer that works for the question)

Edit:

I've unscreened the answers. You had lots of good answers, including a couple I weren't expecting, I've added my answer to the comments below (feel free to add your answer before you view the comments).