How the rate and tax numbers behind these calculators are sourced, checked and corrected.
Every calculator on this site runs on assumptions someone had to choose: a default interest rate, a state sales tax figure, a depreciation curve. This page explains where those numbers come from, how we check that the math itself is right, and what happens when a reader tells us something is off.
Loan and lease calculators default to an interest rate drawn from published national averages for new and used auto financing, most often the Federal Reserve's consumer credit series and rate surveys from major auto lenders. Sales tax defaults reference published state and local tax rates; because rates change and vary by county, the sales tax calculator lets you override the default and enter your own local rate rather than trusting a single number for the whole country. Depreciation guidance in the articles (the 20% first-year figure, the 30-60-90 rule) reflects widely cited industry patterns from resale and valuation data, presented as a planning range rather than a prediction for any specific vehicle. None of this is a live feed. Rates and tax tables are checked and refreshed periodically, not pulled in real time, so always confirm the number that actually matters for your purchase with your lender, dealer or state DMV before you sign.
Before a calculator ships, its formula is checked by hand against a small set of known inputs and outputs. The auto loan and car payment tools use the standard amortization formula (payment equals principal times the monthly rate compounded over the term, divided by that same factor minus one), which we verify against textbook examples and against outputs from other published loan calculators using the same inputs. The total cost of ownership and depreciation tools are checked the same way against manually worked spreadsheets. When a reader reports a number that looks wrong, we rebuild that exact scenario by hand before touching any code, so we know whether the bug is in the formula, the rounding, or the assumption.
Nothing on CarCalcTools is a loan offer, a binding quote, or tax advice. A calculator result is only as good as what you type into it, and real-world figures (your actual APR, your state's exact tax rate, your specific insurance premium) will differ from any default. We say this directly on every calculator page rather than burying it in a footer, because it changes what the number in front of you is useful for: planning a budget, not signing a contract.
If you find a formula error, a stale rate, or a factual mistake in a guide, tell us through the contact page with the page and the specific number in question. We check the report against the original source, fix the page if the report holds up, and note the date of the fix in the article's "updated" line. We do not run reader corrections without checking them first, and we do not remove a correction request just because it is inconvenient to fix.
Calculator guides and articles are written by Jessica Martinez, our contributing writer on car buying and consumer finance. Chris Terry owns and publishes the site through Encore Promotional Products and does not write or edit the calculator content; his role is technical and business operations, not editorial. See the authors page for both roles in full.