Fleet fuel budgets

A fuel budget set in the spring is only as good as the pump price in the autumn. The THRIVE Fleet Fuel Bill Index gives a fleet one number to check it against: the official weekly UK diesel price rebased to 100 at the index's base week, the week commencing 20 July 2026. In the week ending 21 September 2026 the index stood at 117, so a fleet buying the same volume of diesel as it did in July is paying 17% more for it. The week before it was 114.1.

The index moves with the pump price it is built from: 195.53p a litre for diesel in the latest week, up 4.81p on the week, with petrol at 172.01p, up 3.87p. Both series come from the Department for Energy Security and Net Zero's weekly road fuel prices and are on the fuel prices page with their history and a free API.

The bill only tells half the story for a fleet deciding what to run. The Fleet Cost Index sets the diesel cost per mile, 24.69p in the latest week on a documented 36mpg representative van, against HMRC's advisory electric rates of 7p a mile at home and 15p on the public network. At 2.63, the index says a diesel van costs 2.63 times as much to fuel per mile as an electric one on a blended charging mix, which is the number that decides whether a fuel budget is a diesel problem or a charging problem.

Reimbursement follows the same prices with a lag: HMRC's advisory fuel rates are reset each quarter from recent pump prices, and the approved mileage allowance for staff using their own cars rose to 55p a mile for the first 10,000 business miles in 2026/27, the first change to the headline rate since 2011. Both are on the mileage rates and AFR guide.