The cheapest station is often not the best one. Driving eleven kilometres to save two cents costs more than it saves, and no app tells you that. Tanica subtracts the detour from the saving and ranks on what is left.

• 2,459stations, read on 25 September
• 5 minbetween readings
• $2.99once, for Pro

The saving, minus the detour

Price difference times the litres you are actually putting in, minus the fuel the detour burns — and the detour is burned at the price of the station you are driving to, not the one you left. Below twenty-five cents of net gain, Tanica does not pretend you found a deal.

On my way

Give it a destination and it looks in a three-kilometre corridor along the route, then asks Apple Maps for the real detour — there and back onto your route — for the best candidates. The reference is the cheapest station that costs you almost nothing to reach.

Membership stations, counted honestly

A station that requires a membership is marked as such and left out of the best pick unless you say you are a member. A price you cannot buy is not a price.

On the wrist, on the Home Screen, in the Dynamic Island

A standalone Apple Watch app that fetches the data itself, two widgets, a Live Activity that follows a trip, and Siri. A CarPlay screen is written but dormant: it needs a permission from Apple that has not been requested yet.

Price alerts, on the device

Set a threshold on a station and the phone checks in the background, at most once an hour, and tells you when it drops. Nothing about the alert leaves the phone — there is no list of your watched stations anywhere but on it.

What leaves, and who receives it

Our relay receives nothing but the fingerprint of the copy you already have: the whole country's prices come down, and every calculation happens on the phone. Apple Maps is the other side of that coin — it receives your starting point, your destination, what you type in the search field and the candidate stations, because that is what computing a route means. The privacy page says so in those words.

Free, and what Pro adds

The map, the list, favourites, the net saving itself, the widgets, the Watch and Siri are free, with no ads and no account. Pro adds On my way, the CarPlay screen, the Live Activity and price alerts. $2.99 once, shared with your family.

Where the prices come from

The Régie de l’énergie du Québec publishes them; Tanica reads that file every five minutes and is not affiliated with the Régie. If the file comes back short, the last good one is kept rather than served broken.

Seven languages, nine variants: English, French, German, Spanish (Spain and Latin America), Italian, Japanese and Portuguese (Brazil and Portugal).

Tanica is at version 0.1 and has not been submitted. The relay has been live since 25 September. There is nothing to download yet — this page will carry the link when there is.