August's grind became a surge. Across the Costco filling stations we track in the UK, the average price of unleaded finished September at 166.7p a litre — up 12.5p, or 8.1%, from 154.2p on September 1, and diesel rose harder still, up 16.5p (+9.5%) to 191.0p. Both closed the month at their high, and every one of the 22 stations we track got more expensive. Nationally, the RAC reported that diesel had reached a record high of 199.18p on September 28, beating the peak set after Russia's invasion of Ukraine. It is more than three times the rise in the August report, and it sits next to September's moves in the U.S., Canada, and Australia.
| Grade | September 1 | September 30 | Month change |
|---|---|---|---|
| Unleaded petrol | 154.2p | 166.7p | ▲ 12.5p (8.1%) |
| Super unleaded | 165.5p | 176.2p | ▲ 10.7p (6.5%) |
| Diesel | 174.5p | 191.0p | ▲ 16.5p (9.5%) |
National averages of posted prices at tracked UK Costco filling stations, p/L. All 22 stations posted throughout the month; there is no snapshot for September 18. Snapshot of calendar September 2026 — this article is not updated.
Up almost every day, all month
There was no pause this time. Petrol fell back on only two days all month — a fraction of a penny on September 6 and again on September 26. The steepest stretch came mid-month: unleaded went from 156.1p on September 8 to 162.3p by September 17, more than 6p in nine days. Diesel did the same from 177.1p to 184.0p, then added another 7p over the last fortnight to close at 191.0p. The gap between the two widened from 20.3p to 24.3p a litre, because diesel was where the supply squeeze bit hardest. Super unleaded followed the same path, up 10.7p to 176.2p.
Every station rose — most by double digits
For the second month running there is no fallers table. Reading, Leicester, and Aberdeen each added 15pto unleaded, and six more stations — Bristol, Coventry, Derby, Edinburgh, Oldham, and Watford — added 14p. The smallest rise was Liverpool's 6p, which left it as the cheapest unleaded in the network on September 30 at 158.9p, 4p clear of the next-cheapest, Haydock and Leeds at 162.9p. Reading is the priciest on its own at 171.9p, ahead of a five-way tie at 168.9p: Bristol, Coventry, Gateshead, Leicester, and Thurrock. The spread between cheapest and priciest widened from 5.0p at the end of August to 13.0p, so which station you use matters a lot more than it did a month ago. On diesel, Leicester rose the most (+21p), and Edinburgh and Watford were cheapest at 184.9p while Reading was priciest at 196.9p.
| Station | Unleaded now | Month change |
|---|---|---|
| Reading | 171.9p | ▲ 15.0p |
| Leicester | 168.9p | ▲ 15.0p |
| Aberdeen | 166.9p | ▲ 15.0p |
| Bristol | 168.9p | ▲ 14.0p |
| Coventry | 168.9p | ▲ 14.0p |
| Derby | 167.9p | ▲ 14.0p |
| Edinburgh | 165.9p | ▲ 14.0p |
| Oldham | 165.9p | ▲ 14.0p |
| Watford | 165.9p | ▲ 14.0p |
Biggest September rises in posted unleaded at tracked UK Costco filling stations, September 1 – 30, 2026. All 22 stations rose.
Cheapest unleaded right now
| Station | Price |
|---|---|
| Liverpool | 158.9p |
| Haydock | 162.9p |
| Leeds | 162.9p |
| Manchester | 163.9p |
| Birmingham | 164.9p |
Posted unleaded prices at tracked UK Costco filling stations, September 30, 2026.
Why September was so steep
It was oil, and specifically diesel. On September 4 the RAC said Brent was above $94 a barrel and warned that drivers would soon pay noticeably more. By September 14 both fuels had risen 8p since the start of the month and the RAC was reporting a new Iran-war high, with oil "well over $100 a barrel". Diesel took the brunt of it. PetrolPrices reported wholesale diesel up more than 12p a litre in September by mid-month, driven by a tight distillate market and the suspension of Saudi Arabia's East–West pipeline, one of the few routes that bypass the Strait of Hormuz. Then on September 28 Brent jumped toward $108 a barrel after Washington rejected Iran's offer to reopen the strait within a week.
Tax was not part of it. The 1p fuel duty rise planned for September 1 was cancelled in May, so duty stayed at 52.95p a litre all month. With duty unchanged, the entire 12.5p petrol and 16.5p diesel rise at Costco came from the cost of the fuel itself, plus the VAT charged on it. Duty and VAT still make up roughly half of every litre, which is why an oil shock this size shows up as an 8.1% petrol and 9.5% diesel rise at the pump rather than something larger.
Costco is still well below the national average
On September 28 the RAC put the UK average at 174.13p for petrol and 199.18p for diesel. Costco's national averages that day were 165.8p and 189.8p, about 8p and 9p cheaper. The RAC's September rise (nearly 12p on petrol and 14.5p on diesel by September 25) was similar to Costco's, so the discount held. Prices went up everywhere; Costco went up from a lower starting point.
Will fuel prices fall in October?
Not quickly. The RAC's view is that only a sustained lower oil price — over several weeks, not days will bring pump prices down, and UK forecourts take up to a fortnight to pass on a wholesale change in either direction. The RAC called talk of a renewed deal to end the Hormuz blockade "a slight glimmer of hope", but the late-September jump in oil had not yet reached the pumps when the month closed. Costco ended September at its high on both grades, so the start of October is likely to be higher still, not lower. Duty is frozen until the end of the year. For now, Liverpool at 158.9p is the cheapest unleaded on the network by a clear 4p, and Edinburgh and Watford are the cheapest diesel at 184.9p; Reading is the most expensive for both.
Costco pump prices are a snapshot of calendar September 2026 (September 1–30) and will differ at the pump today. This site is independent and not affiliated with Costco Wholesale Corporation.
