Car Loan Payoff Calculator Overlay

WHAT IT DOES

This userscript helps car owners understand payoff options by projecting loan timelines and interest savings. It scans car loan pages, auto loan calculators, or bank websites for loan details—loan amount, interest rate, loan term, monthly payment, and remaining balance. It displays an interactive calculator panel to project payoff timelines and interest savings, extract figures from text, calculate the impact of extra payments, show the total interest saved, and display a payoff timeline. The script compares different payment strategies, shows the effect of rate changes, highlights the fastest payoff strategy, and exports the analysis as CSV or HTML. It also suggests payment amounts to pay off the loan by a target date.

WHERE IT RUNS

The script works on car loan pages, auto loan calculators, bank websites, and any page with loan data. It runs entirely within your browser, reading only the visible content of the current page. It does not track your browsing history, access other tabs, or send any data externally. All loan data is processed and stored locally.

HOW TO USE

After installation, open the panel by clicking the tool's icon in the BotGentz sidebar. Enter your loan amount, remaining balance, interest rate, loan term, monthly payment, and any extra payment. Click "Calculate Payoff" to see monthly payment, total interest, payoff time, and total cost. If you add extra payments, the script shows how much interest and time you save. A payoff timeline bar chart shows the remaining balance over time. Click "Calculate payment for target payoff date" to find out how much you need to pay each month to pay off the loan by a specific date. Use "Extract from page" to auto-fill loan details. Use the export dropdown to choose CSV or HTML. Click "Export" to download.

THE TECHNICAL MECHANISM

This script uses standard loan amortization to calculate payoff projections. The monthly payment is either user-provided or calculated using the formula: P = principal × (r(1+r)^n) / ((1+r)^n - 1), where r is the monthly interest rate and n is the number of months. The payoff simulation runs month-by-month: interest is calculated on the remaining balance, payment is applied, and the remaining balance is reduced by the principal portion. Extra payments are added to each monthly payment, accelerating the payoff. The script tracks total interest, total paid, and remaining balance at annual intervals for the timeline chart. The target date calculator solves for the monthly payment needed to pay off the loan in a specified number of months. All data is persisted using GM_setValue.

THE PANEL

The panel is draggable anywhere on the screen and remembers its position between sessions. It can be collapsed to a small floating bubble to minimize distraction, and expanded again with a single click. The panel's width can be adjusted, and pressing the Escape key closes it instantly. All settings and loan data are persisted locally using GM_setValue, so your calculations remain intact across page reloads and browser restarts.

PLEASE NOTE

This userscript requires the free BotGentz extension to function. The script extracts figures from page text—it may not capture all numbers correctly. Review and adjust extracted data as needed. The calculations are estimates based on the data provided and do not account for all loan fees or rate changes. The script does not access external APIs or store any data outside your browser. No data from your browsing is stored or transmitted anywhere—all processing occurs entirely in your browser.