How can I get my interest rate lowered on my auto loan?

Can I call and ask for the loan to be refinanced? This was my first vehicle I bought by myself with no co-signer. Since my previous cars' loans were in my mom's name and I was only a co-signer, I had no car credit when I bought my vehicle now. Therefore my interest rate was a bit higher around 8%. I don't want to extend my payments or anything. I just want a lower interest rate. I bought this vehicle in Nov of 06. I've consistently made my $600/month payment on time. I would think after a year and a half that i have now established some car credit. Is it feasible to call up my auto loan company and ask them? Has anyone else done something like this before?