Monthly auto loan interest calculation

Aug 2009
356
0
I am trying to find a faster way of calculating my monthly interest on a auto loan that I have. The remaining balance of my loan is $8400 with an Apr of 4.94%. My monthly payment is $716.94. But I want to make 4 weekly additional payments of $500 to speed up process of paying off my loan. So I have been using I= prt to calculate daily interest and adding up the weeks to get my total for the month. But since my additional payments are weekly is there another way I could do this calculation faster?
 

romsek

MHF Helper
Nov 2013
6,748
3,037
California
I am trying to find a faster way of calculating my monthly interest on a auto loan that I have. The remaining balance of my loan is \$8400 with an Apr of 4.94%. My monthly payment is \$716.94. But I want to make 4 weekly additional payments of \$500 to speed up process of paying off my loan. So I have been using I= prt to calculate daily interest and adding up the weeks to get my total for the month. But since my additional payments are weekly is there another way I could do this calculation faster?
escaped dollar signs for readability
 
Feb 2015
2,255
510
Ottawa Ontario
Futility exercise!
With no extra payments, total interest left on loan = ~227 dollars
Extra 2000 dollars per month reduces above to ~72 dollars
Useless to calculate "to the penny"!