Vehicle EconomicsCo-Signer Cost and Risk Calculator
See both the interest savings a co-signed rate produces and the co-signer's own real dollar exposure.
Why this comes up
Co-signing lowers the primary borrower's rate, but the co-signer is asking a real question most calculators skip: what exactly am I on the hook for? This engine shows both sides at once, the interest a co-signed rate saves versus borrowing solo, and the co-signer's own peak dollar liability if the loan is never paid down.
How we calculated this
We run a real amortization schedule at the co-signed rate to get total interest and the month-by-month loan balance. Peak liability is simply the starting loan balance, the most a co-signer could ever owe, since the balance only goes down from there. If a solo rate is supplied, we run a second schedule at that rate and report the interest difference.
Worked example, using this page's own defaults ($24,000, 6% co-signed vs 14% solo, 60 months): run the calculator above to see the exact interest saved and the co-signer's peak liability.
What this means
- Peak liability is highest on day one and shrinks every month the loan is paid on time, but it does not shrink at all if payments are missed.
- A co-signer's exposure is legal and full, not proportional, missed payments can be pursued against the co-signer for the entire remaining balance.
- A shorter term lowers peak liability duration even if the monthly payment is higher.
Limitations
This is not legal advice on co-signer liability, which varies by lender and state; it models the loan math only. See Methodology.
Methodology
This engine uses Vehicle Economics' fixed-rate amortization model to build the full month-by-month schedule both the interest comparison and peak liability are drawn from. See Methodology for the full detail.