138,700 btu's
Oil has the higher BTU rating Depends. If it is Liquified Nat. Gas then it has the higher rating.
The energy content in fuels is often measured in BTU (British Thermal Units). I'm assuming that you're talking about compressed natural gas, as in the form that is used in some cars like the Honda Civic GX. One gallon of compressed natural gas contains 138,700 BTU. One gallon of gasoline contains 125,000 BTU. [http://www.bts.gov/publications/national_transportation_statistics/2002/html/table_04_06.html]
natural gas
The BTU (British Thermal Unit) rating of natural gas typically ranges from 950 to 1,100 BTUs per cubic foot. This measurement indicates the energy content of natural gas and is used to determine its heating value.
114,000 BTU/gallon
For the purpose of gas rating 1040 btu's/ft3
One cubic meter of compressed natural gas (CNG) typically contains between 9000 to 11000 BTUs (British Thermal Units) of energy.
For regular gasoline, 125,000 BTU per US gallon
140,000 btu/gallon, nominal
114100 Btu
In a gallon of gasoline, there is approximately 124,884 btu. BTU refers to British thermal unit. It is actually a traditional unit.
Propylene has a lower heating value of approximately 21,500 BTU per gallon.