Maths is putting numbers into logic and rationality. If you can reason out the problem, then it is already partially solved on its own.

You already got the answer on your own. Either using the online calculator or the excel worksheet shown in above post, it is about $51,000 to settle the car loan. And the savings in interest cost is about $5700 or $5954 from either calculations.

This $51k is taken from the flexi loan, which if it were to remain there, it would earn interest of 4.15%.
If it remains there for the other 46 months, it will get a return of 51,000 x 4.15% x 46 / 12 = 8113.25

So, you save $5700 (or $5954), but lose $8113.

Why you would lose more than you saved? Because the cost of interest in using the loan facility is not in proportion to its usage. You are paying much more interest at the beginning.

(It is like travelling on a toll highway that charges you a very high fee per mile for the first several miles and gradually reduces the fee as you travel further.)

Actually the math don't end here. You can continue on further with the numbers. Since the car loan is fully settled, and no longer need to pay the car installment every month, $1235 is freed and can be placed into the flexi account to earn interest.

If this is the case, then the interest earned would work out to be about $4865.

So, the final figure: 5700 - 8113 + 4865 = 2452.

You gained $2452, over a period of nearly 4 years.

Chump change when compare to the total salary earned in 4 years.