Gasoline contains about 34.6 megajoules per litre (MJ/l) or 131 MJ/US gallon. This is an average; gasoline blends differ, therefore actual energy content varies from season to season and from batch to batch, by as much as 4% more or less than the average, according to the US EPA.

Volumetric energy density of some fuels compared with gasoline:^[7]

(*) Diesel is not used in a gasoline engine, so its low octane rating is not an issue; the relevant metric for diesel engines is the cetane number

A high octane fuel such as LPG has a lower energy content than lower octane gasoline, resulting in an overall lower power output at the regular compression ratio an engine ran at on gasoline. However, with an engine tuned to the use of LPG (ie. via higher compression ratios such as 12:1 instead of 8:1), this lower power output can be overcome. This is because higher-octane fuels allow for a higher compression ratio - this means less space in a cylinder on its combustion stroke, hence a higher cylinder temperature which improves efficiency according to Carnot's theorem, along with fewer wasted hydrocarbons (therefore less pollution and wasted energy), bringing higher power levels coupled with less pollution overall because of the greater efficiency.

The main reason for the lower energy content (per litre) of LPG in comparison to gasoline is that it has a lower density. Energy content per kilogram is higher than for gasoline (higher hydrogen to carbon ratio). The weight-density of gasoline is about 737.22 kg/m³.

Different countries have some variation in what RON (Research Octane Number) is standard for gasoline, or petrol. In the UK, ordinary regular unleaded petrol is 91 RON (not commonly available), premium unleaded petrol is always 95 RON, and super unleaded is usually 97-98 RON. However both Shell and BP produce fuel at 102 RON for cars with hi-performance engines, and the supermarket chain Tesco began in 2006 to sell super unleaded petrol rated at 99 RON. In the US, octane ratings in fuels can vary between 86-87 AKI (91-92 RON) for regular, through 89-90 (94-95) for mid-grade (European Premium), up to 90-94 (RON 95-99) for premium unleaded or E10 (Super in Europe)