Using the saving calculator

The calculator converts a published difference into pounds. You type the size of your tank in litres; it multiplies the difference between two published area medians by that volume and divides by one hundred to give pounds. Nothing else happens, and nothing you type leaves your browser.

It can only work on figures that exist. While no window is complete, there is no published area median, so the calculator has nothing to multiply and says so. When medians are published, the calculator uses two areas and one fuel grade from the same window, because a difference between two grades or two windows would not describe a real choice.

What it gives you is arithmetic on history, not a forecast. Prices move between the window and the moment you fill up, and a median is not an offer from any station. A saving figure also ignores what it costs to reach a different area.

Two habits keep the result honest. Use the same fuel grade on both sides, and check how complete each window is before trusting a small difference. For the price right now, use the Fuel Finder service run by the department, linked on every page.