Here is one way that seems to work:

Payment: 188.71 + 500 = 688.71

=RATE(16,-688.71,10000,0,0,0.01)

produced: 0.0116550291317306

or 1.17% This is the per period interest. the Annual would be 12 times
that:
13.986%

If I check it as
=PMT(0.0116550291317306,16,10000)

I get a payment of 688.71


RATE
Returns the interest rate per period of an annuity. RATE is calculated by
iteration and can have zero or more solutions. If the successive results of
RATE do not converge to within 0.0000001 after 20 iterations, RATE returns
the #NUM! error value.

Syntax

RATE(nper,pmt,pv,fv,type,guess)



--
Regards,
Tom Ogilvy


"lawdoggy" wrote:

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