Used Car Listing Mileage-per-Year Calculator

WHAT IT DOES This userscript helps used car buyers evaluate vehicle wear and longevity by calculating the average miles driven per year. It scans used car listing pages—Autotrader, Cars.com, Craigslist, Facebook Marketplace, eBay Motors, and similar sites—to extract the vehicle year and mileage. It then calculates the average annual mileage and compares it to the US average (12,000–15,000 miles/year). The result is displayed as a visual badge or panel showing whether the vehicle has Below Average, Average, Above Average, or High mileage. The script also estimates how many years the vehicle may have left based on a typical lifespan of 200,000 miles, projects mileage over the next 5 years, and suggests maintenance items based on current mileage thresholds. WHERE IT RUNS The script works on used car listing pages—Autotrader, Cars.com, Craigslist, Facebook Marketplace, eBay Motors, and similar classified sites. 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 calculations are performed locally. HOW TO USE After installation, open the panel by clicking the tool's icon in the BotGentz sidebar. The script auto-scans car listing pages on load (toggle this on/off). Choose your display mode—"Badge only" shows a floating badge in the corner, "Panel only" shows details in the panel, or "Both" shows both. Select your average mileage baseline—12,000 mi/year (Conservative), 13,500 mi/year (US Average), or 15,000 mi/year (High). Click "Analyze mileage" to manually scan the page. The badge displays the status, annual mileage, year, mileage, age, and estimated remaining life. The panel shows a full breakdown including status description, vehicle details, remaining lifespan estimate, 5-year mileage projection, and maintenance suggestions based on current mileage. Use "Clear analysis" to remove all overlays. THE TECHNICAL MECHANISM This script uses text extraction and arithmetic analysis to evaluate vehicle mileage. It scans the page for year and mileage using regex patterns—matching \b(19|20)\d{2}\b for years and ([\d,]+)\s*(?:miles|mi|mile) for mileage. It also searches common listing selectors like .details, .description, and [class*="mileage"] for additional data extraction. The script calculates the vehicle's age (current year minus model year) and average annual mileage (total mileage divided by age). It compares the result to the user's selected baseline and categorizes it: Below Average (180%, red). The remaining lifespan is estimated by subtracting current mileage from 200,000 miles and dividing by the annual rate. 5-year projections are calculated by adding the annual rate each year. Maintenance suggestions are based on mileage thresholds: 30k (brakes), 45k (tires), 60k (timing belt), 75k (transmission fluid), 90k (spark plugs), 100k (major service), and 120k (cooling system). 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—display mode, average baseline, and auto-scan—are persisted locally using GM_setValue, so your preferences remain intact across page reloads and browser restarts. PLEASE NOTE This userscript requires the free BotGentz extension to function. The script is designed for used car listing sites; it may work on other classified sites but is not guaranteed. The extraction relies on the current text patterns on each site—if a site updates its UI, the script may need to be updated. The lifespan estimate (200,000 miles) is a general average; actual vehicle longevity varies by make, model, maintenance history, and driving conditions. Maintenance suggestions are general recommendations; always consult a mechanic for specific advice. The script does not access your account or API; it only reads what is visible on the page. No data from your browsing is stored or transmitted anywhere—all processing occurs entirely in your browser.