VydajeZaAuto.cz
Own product
A web product I built and run: calculators for vehicle running costs, with the calculation methods published openly.
Context and problem
What a car really costs is spread across many items: loss of value, fuel, insurance, service, tyres, the motorway vignette, and smaller costs. It is hard to see them together as one monthly figure or a price per kilometre, and harder still to compare decisions such as repairing or selling, a loan or leasing, or one fuel against another. I wanted calculators for these decisions about my own car, did not find suitable ones, and built them. VydajeZaAuto.cz is now an independent site that helps Czech drivers make such decisions with numbers.
My role
I built the whole product on my own, working with AI tools: the calculation methods, the interface, the content, and its operation.
Approach
- Methods in the open. A methodology page explains each calculator, and a shared catalogue lists every assumption with its value, unit, source, and revision date.
- Defaults you can change. Calculators start from Czech market estimates, and the user can overwrite any of them with their own numbers.
- Inputs stay on the device. Values entered into a calculator are kept only in the browser. Analytics run only after consent and never receive financial values or vehicle details.
- Clear limits. Every result is presented as an estimate, not an offer or financial advice, and the methodology says what the calculators cannot know.
- Checked against reality. The formulas of each calculator are tested on their own with fixed expected results, browser tests walk through every calculator, and I checked the default values against real Czech prices. Fuel prices come from the European Commission’s Weekly Oil Bulletin.
What was delivered
Eight calculators:
- total cost of ownership per month, per year, and per kilometre, with a share link
- repair or sell
- commuting costs
- future resale price
- loan or leasing for a new car
- the best age of a car to buy
- petrol, diesel, LPG, CNG, or electricity
- the price of comfort compared with public transport
Alongside them: the methodology, FAQ, explanatory articles, and privacy and terms pages.
Outcome and status
The product is live with eight calculators and published methods, and I keep developing it. It has been live since April 2026.
Technical notes
Next.js and React in TypeScript, hosted on Vercel, with no database: the entered values live in the link and in the browser, which is also what makes results shareable. The formulas are kept apart from the interface so they can be tested on their own, and the default values have a single source that also feeds the published list of assumptions, so the site and its methodology cannot drift apart. Every change runs through the tests before it is deployed.