How JoltMile works out your fuel cost

What it is

An estimate of what each shift cost you in fuel or charging, calculated from your vehicle rather than from receipts.

Why JoltMile does it this way

Fuel receipts do not line up with shifts. You fill up on Tuesday and that tank covers Wednesday, Thursday and half of Friday, including a school run. Working from cost per mile gives you a per-shift figure that receipts cannot.

How it is calculated

For petrol, diesel and self-charging hybrid:

cost per mile = (4.546 ÷ your MPG) × price per litre

4.546 is the number of litres in an imperial gallon. For electric it is price per kWh ÷ miles per kWh. For a plug-in hybrid, JoltMile blends the two using the battery-share percentage you set - the proportion of your driving you do on electricity.

That cost per mile is then applied to total miles for the shift cost, and to business miles alone for the business-only figures. Bicycle shifts have no fuel cost at all.

Where the price comes from

JoltMile fetches the UK average pump price weekly from the government's published fuel price data, and shows the source's own publication week - "UK average, week commencing 17 August 2026" - rather than the date we fetched it. Our fetch date would make a week-old figure look like today's on six days out of seven.

Prices are sanity-checked to a plausible range. A value outside it is treated as a parsing failure rather than a cheap week: JoltMile keeps the last known good price with its original date and logs the problem, instead of quietly storing something wrong.

You can override the price at any time in Settings. Once you do, the weekly refresh leaves your figure alone until you reset it to the average.

Every shift keeps its own price

When you log a shift, JoltMile saves the fuel price, MPG, battery share and mileage rates that were in force at that moment onto that shift. Every figure the shift ever produces is worked out from its own saved copy, not from whatever your settings say today.

This is the rule, and it has no exceptions:

• Changing a price in Settings affects shifts you log from now on.
• It never re-costs a shift you have already logged, and never moves a past month's fuel, current balance, hourly rate or tax estimate.
• Editing an old shift - even changing its notes - does not re-price it either. It keeps what it was logged at.

That is what makes last March comparable with this March. If a price rise reached backwards through your history, every figure you had already looked at would change under you, and you would never be able to tell a bad month from an expensive one.

Mistakes to avoid

• Using the manufacturer's MPG. Delivery driving is stop-start with a loaded car; measure your own.
• Changing your Settings price to fix an old shift. It will not - correct the shift itself.
• Setting a price per litre from a supermarket promotion and forgetting it. The reset-to-average button is always available.
• Expecting it to match your receipts exactly. It is a good estimate of the cost of the miles you drove, not an accounting of the fuel you bought.