I need help with the formula for figuring out what the 'real' interest
rate is when you pay a loan off early. Example car loan:

$10,000 (borrowed) 5% @ 60 months, Total with interest is: 11,322.74

Now:
I do the same as above but apply an extra $500 to the principle each
month and pay off the loan in 16 months and only pay $336.75 in
interest. What is my new 'real' interest rate for this loan?

Thanks in advance for anyone who can help! mitch