Seven weeks, one number: the diesel-to-electric gap keeps widening

Since 20 July we have rebuilt the same figure every Tuesday: how many times more expensive a diesel van is to fuel, per mile, than an electric one. It started at 2.24 times. Last week it hit 2.50 times. It has not fallen once.

That is seven consecutive weekly readings moving in the same direction, and it is worth sitting with for a moment, because the underlying inputs have not changed. HMRC's advisory electric rate has been stuck at 7p a mile for home charging and 15p for public since the split came in back in June. The electric side of the equation has been flat all summer. Every bit of the movement has come from the diesel side.

The series

Source: THRIVE Fleet Cost Index, built from DESNZ weekly road fuel prices and HMRC advisory fuel rates under the Open Government Licence v3.0.

Diesel has gone from 21.10p a mile to 23.53p a mile in seven weeks. That is 2.43p, or an 11.5 per cent rise in the cost of moving a van a mile, over a period most fleets would describe as uneventful.

What 2.43p a mile actually is

Per mile it is nothing. Scaled, it is a line item.

A single van covering 20,000 miles a year picks up roughly £486 of additional annual fuel cost from that seven-week move alone. A fifty-van operation covering the same mileage is looking at about £24,300. A two-hundred-van operation is at £97,200. Nobody sent an invoice, nobody approved a price increase, and in most fleets nobody has noticed yet, because fuel card spend gets reviewed monthly against a budget that was set in the spring.

That is the real argument for tracking a single derived number weekly rather than reading pump price headlines. The headline tells you diesel moved. The index tells you what it did to your cost per mile, which is the unit your budget is actually built in.

The break-even that matters more

At this week's reading, an electric van repays its price premium over a diesel equivalent at 12,739 miles. That assumes an annualised premium of £1,800 and the blended charging split described in our methodology - and it is worth being straight about the weaker half of that sum. The £1,800 is a documented placeholder while we source a defensible figure, and the break-even scales linearly with it, so treat the mileage as indicative rather than settled. The direction is not in doubt; the precise threshold is.

Twelve thousand seven hundred miles is not a stretch target. It is under two-thirds of the annual mileage of a typical multi-drop urban van. For any vehicle on a genuinely high-mileage duty cycle, the running-cost case stopped being marginal some time ago and the remaining objections are about charging access, payload and residual risk, not fuel.

The corollary is just as useful, and less often said out loud. For a van doing 6,000 miles a year - a site vehicle, a rarely-used spare, a specialist unit - the fuel arithmetic does not get you there, and no amount of index movement will. Fleets that electrify by blanket policy rather than by duty cycle are paying a premium on exactly the vehicles least able to earn it back.

Your advisory fuel rate is probably stale

The other consequence is reimbursement. HMRC's advisory fuel rates are reviewed quarterly. The index has moved 11.5 per cent in seven weeks. If your diesel drivers are being reimbursed at a rate set before this run began, they are out of pocket on every business mile, and you will hear about it eventually - usually at the point where somebody stops claiming and starts avoiding the journeys.

Two practical checks, neither of which takes long:

1. Pull your current reimbursement rate and compare it against the most recent published AFR rather than the one you adopted. The gap between "the current rate" and "the rate we set up in the system" is where most of the friction lives.
2. If you reimburse electric drivers, check whether you have implemented the home and public split at all. A single blended rate was defensible before June. It is not now, and it systematically underpays drivers who cannot charge at home - which tends to correlate with the drivers who can least absorb it. Our charging cost gap page holds the current 7p-versus-15p spread.

What we will be watching

Three things would break the trend. A meaningful fall in wholesale diesel, which nothing in the current market points to. An upward revision to the advisory electric rate, which would narrow the gap from the other side and is plausible at the next quarterly review. Or a change to the way the rates are structured, which is a live possibility given how recently the split was introduced.

Until one of those happens, the number goes up. We publish it every Tuesday, the underlying data is free to download, and the API needs no key.