LPG conversion in the UK in 2026: the payback period maths at current p/L prices

What a conversion actually costs in 2026

A professional LPG conversion from a DVSA-recognised LPGA-approved installer typically runs £1,800 to £2,500 for a four-cylinder petrol car, and £2,500 to £3,500 for a six-cylinder or larger engine. The work involves fitting a separate LPG tank (usually in the spare wheel well or boot), a vaporiser, dedicated injectors and an ECU that switches between fuels. Direct-injection petrol engines are more complex and sometimes not supported at all.

On top of the install, you should budget for a specialist service every 20,000 miles or so, and factor in that resale value gains are modest — most used-car buyers see a bi-fuel conversion as neutral at best. Insurance must be notified, though premiums rarely move materially. Treat the headline conversion price as the floor, not the ceiling.

The per-mile maths at current pump prices

LPG contains roughly 25% less energy per litre than petrol, so a car that returns 40 MPG on unleaded will typically manage around 32 MPG on autogas. That matters: the pump price gap shrinks once you account for consumption. Using indicative 2026 prices — petrol at 135p/L and LPG at 95p/L, broadly in line with DESNZ weekly road fuel data and AA Fuel Price Report trends — a 40 MPG petrol car costs about 15.3p per mile to fuel. The same car on LPG at 32 MPG costs about 13.5p per mile.

That is a saving of roughly 1.8p per mile, or about £180 per 10,000 miles. For a 15,000-mile-a-year driver the gross annual saving sits near £270. Against a £2,000 conversion, simple payback lands at about seven to eight years. Push the mileage to 25,000 a year and payback falls below five years; drop to 8,000 miles and it stretches past a decade.

The hidden variables that break the model

Fuel duty is the biggest wildcard. LPG currently attracts a much lower duty rate than petrol or diesel, and the OBR has repeatedly flagged alternative fuel duties as a candidate for alignment as petrol receipts erode. Any meaningful rise in LPG duty would compress the per-litre gap and extend payback significantly. The 2026 Budget cycle is worth watching closely.

The second issue is availability. The UK LPG forecourt network has contracted sharply over the past decade and now sits at roughly 1,100 to 1,200 sites, concentrated along motorway corridors and in the Midlands and North. Rural drivers in the South West, Wales or Scotland may find themselves routing journeys around autogas pumps, which adds detour miles and cognitive load.

Finally, resale timing matters. If you plan to keep the car for three years and sell, the maths rarely works. If you intend to run a high-mileage estate or 4x4 into the ground over a decade, it can.

Who should actually consider it

LPG conversion makes most sense for a specific profile of driver, and poorly for everyone else. The sweet spot is a high-mileage user running a larger-engined, port-injected petrol car that is otherwise expensive to feed and not easily replaced by an EV.

The groups most likely to see genuine payback include:

The verdict for 2026

At today's prices the economics are workable but not spectacular. Payback periods of five to eight years assume stable fuel duty, a working local LPG pump and a car you plan to keep. Change any one of those and the business case weakens quickly.

For most private motorists doing average mileage in a modern small petrol car, the £2,000 is better spent elsewhere — on tyres, a cautious switch to a used hybrid, or simply held in reserve. For the right high-mileage driver with the right car, LPG remains one of the few genuinely cheaper ways to cover ground on four wheels in the UK.