Vehicle data: NHTSA vPIC · Tax rules: IRS · Data updated: July 17, 2026 · Corrections: editorial policy
CarLoanCheckup runs two tools. The VIN checker answers the US final-assembly test. The MAGI calculator runs the income phase-out for the car loan interest deduction (Section 70203, H.R.1, tax years 2025–2028). This page shows how both work. Every figure traces to two sources: the IRS for tax rules, NHTSA vPIC for vehicle data. We are not CPAs or enrolled agents, and we don't pretend to be. The method is the authority here: read the primary sources, publish the formula, log every change.
In March 2026 the IRS released FS-2026-09, a guide to spotting fake tax deduction calculators. We use it as our checklist — the last section maps every warning sign to this site's practice.
How the VIN check works
Enter a 17-character VIN on /check . Your browser calls the NHTSA vPIC API directly — the federal VIN decoder. We read two fields from the reply: Plant Country and plant city. Plant Country names the country of final assembly, which is the legal test. The call is client-side. Your VIN goes from your browser to NHTSA's public database and nowhere else. We never see it, so there is nothing to log. A repeat lookup of the same VIN comes from your browser's session cache, not a second API call.
Client-side by design: your VIN goes from your browser to NHTSA and nowhere else, so there is nothing for us to log.
Why per-VIN? A model name can't settle the question. Chevrolet builds the Silverado in Fort Wayne, Indiana and in Silao, Mexico. Honda builds CR-Vs in Ohio and in Ontario, Canada. Two matching trims on one lot can split — one passes, one fails. A model list gives odds. The VIN answers for your car. Schedule 1-A prints that VIN on the form itself; the IRS can run the same lookup.
The checker names its failure modes instead of guessing. Some vPIC records come back with blank plant data — mostly older VINs and low-volume imports. Fleet and commercial VINs can decode in part only. In both cases the verdict reads "can't verify," never "eligible." And when vPIC disagrees with the Monroney window sticker — rare, but real — the checker won't pick a winner. The IRS hasn't said which document controls. That conflict goes to a tax professional, paperwork in hand.
How the calculator works — the exact formula
/calculator runs the Section 70203 phase-out with nothing added and nothing smoothed.
Step 1. Start at the $10,000 annual cap ( IRS: OBBBA provisions ).
Step 2. Subtract $200 for each $1,000 of MAGI ("or portion thereof") above $100,000 single or $200,000 married filing jointly. A part-thousand rounds up: a MAGI of $120,001 counts as 21 blocks over, and the cap drops to $5,800, not $6,000.
Step 3. Floor the cap at $0. The quoted zero-points are $150,000/$250,000. Strictly, the round-up rule zeroes the cap once MAGI passes $149,000/$249,000.
Step 4. Your deduction is the lesser of that cap and the interest you paid.
No version of the calculator ships until the formula passes its unit tests — one each for the thresholds, the $200 step, the round-up rule, and the $0 floor. The calculator runs fully in your browser. Your numbers never leave your device. The output is labeled an estimate and shows tax savings by bracket, because this is a deduction, not a credit — it trims taxable income, not your bill dollar for dollar.
Our data sources and refresh cadence
| Source | What we use it for |
|---|---|
| NHTSA vPIC. | Plant Country and plant city, per VIN. |
| IRS Schedule 1-A instructions. | How to claim the deduction on Form 1040. |
| IRS OBBBA provisions page. | Who qualifies, the income limits, the $10,000 cap. |
| IRS FS-2026-09 (March 2026). | The fake-calculator warning. Our checklist. |
| IRS Notice 2025-57 (October 2025). | 2025 transition relief. Why season-one filers got lender letters, not Form 1098-VLI. |
The mirror rule: we quote IRS numbers exactly. A secondary source never backs a rule on who qualifies. If a blog and the IRS disagree, the IRS wins and the blog goes uncited.
We check irs.gov monthly for new OBBBA guidance. Every page carries a "Data updated" date. Before each filing season (December 2026, then December 2027) we re-check every rule on the site against the then-current Schedule 1-A instructions, line by line.
IRS guidance change-log
The 2026 guidance is fresh and can still change. This log records every rule change and every fix. The correction policy is short: an error we find, or a reader reports, gets fixed on the page and logged here with a date. How to report one: editorial policy .
| Date | Change | Pages |
|---|---|---|
| July 17, 2026. | First check of all figures against the Schedule 1-A instructions, the OBBBA provisions page, FS-2026-09, Notice 2025-57. | All. |
What we never do
FS-2026-09 lists the marks of a fake deduction calculator. Point by point, against this site:
| IRS warning sign (FS-2026-09) | This site |
|---|---|
| Guarantees a refund or promises a "big payday." | Estimates only. The word "guarantee" is banned site-wide. Typical first-year savings run $350–900, not $10,000 (our estimate; run yours). |
| Collects SSN, date of birth, or bank details. | Zero PII. No signup and no email. The calculator is client-side. The VIN check sends us nothing — the only copy of your VIN is the session cache in your own browser. |
| Vague or unnamed sources. | Two named sources, IRS and NHTSA vPIC, cited inline next to every figure. |
Beyond the checklist: we give no tax advice and no "you should claim" nudges. We don't store VINs or sell tool inputs. No verdict ships without "estimate" or "verify your VIN" language. Where our data can't answer (a vPIC-vs-sticker conflict, say), the page says so and hands off to a professional ( full disclaimer ).
Not tax advice. Sources cited only — expert review pending; consult a licensed tax professional (full disclaimer). Tax rules: IRS (Schedule 1-A instructions; OBBBA provisions; FS-2026-09; Notice 2025-57). Vehicle data: NHTSA vPIC. Data updated: July 17, 2026.
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