Gasoline and fuel economy
Gasolines with higher heating values give better fuel economy than those with lower heating values.
There is generally only a small difference, about 2%, between premium and regular gasoline. This difference traditionally favours starting and warm-up driveability, and this contributes to the poorer fuel economy in winter. Driving style, good vehicle maintenance and driving conditions have a much larger influence on fuel economy than the grade of gasoline.

Driving for fuel economy involves smooth, steady acceleration rather than "jackrabbit" starts; moderate speeds, not carrying heavy loads, not using a luggage rack or towing a trailer unnecessarily, not using an air conditioner or heater unnecessarily, not idling the engine when it could be switched off and avoiding short trips where the engine does not fully warm up.

Maintenance factors include a properly tuned engine, a clean air filter, aligned and balanced wheels, periodic changing of the fuel line filter, and tires with the correct air pressure.

Driving conditions that are beyond our control and reduce fuel economy include cold temperatures, head winds, driving up hills, water, slush, or snow on the road.

Factor Conditions Percent Reduction in
Fuel Economy
    Average Maximum
Temperature -5C vs 25C 5.3 13
Idling/Warm-up Winter vs summer Variable 20
Air Conditioner Extreme heat 21 N/A
Defroster Extreme use Similar to air conditioning
Head Wind 30 kph 2.3 6
Uphill driving 7% grade 1.9 25
Poor road conditions Gravel, curves, slush, etc. 4.3 50
Congested traffic 30 kph vs 45 kph 10.6 15
Highway speed 110 kph vs 90 kph NA 25
Acceleration rate "Hard" vs "easy" 11.8 20
Wheel Alignment 1 cm 1 10
Tire type Radial vs nonradial 1 4
Tire pressure 15 psi vs 26 psi 3.3 6
Windows Open vs closed Unknown but likely small
Office of Mobile Sources, USEPA, August 1995
Gasoline Composition
Gasoline is composed of a mixture of over two hundred hydrocarbons and trace amounts, less than 0.1 volume percent of organic sulphur, nitrogen and oxygen compounds originating from the crude oil or the processing steps to make gasoline; other special chemicals or additives are also added to improve the properties of the gasoline.

The hydrocarbons vary by chemical class - paraffins, olefins, napnthenes and aromatics and within the class by size or molecular weight.

Gasoline is manufactured to meet the physical properties of the specifications and regulations rather than a specific distribution of hydrocarbons. The specifications vary by season and geographical location to ensure adequate performance in the engine and to minimize environmental concerns. To meet the specifications at least cost, complex computer models are used to optimize the blending of the components.

The carbon number distribution of gasoline ranges from C4 to C12 with an average of about C6.8.

Different hydrocarbons have different octane numbers. The octane number varies by class and within a class by molecular weight. Even different isomers (molecular arrangements) of the same hydrocarbon can also have different octane numbers, e.g. the octane number of iso-octane (a highly branched 8 carbon paraffin) is 100, while the octane number of n-octane (a straight chain 8 carbon paraffin) is zero. Other properties are also influenced by the isomer's structure.

Regulations in various locations and jurisdictions limit the amount of volatile compounds, aromatics compounds, olefins and sulphur compounds that are allowed in gasoline.

In some circumstances oxygenates such as methyltertiarybutylether (MTBE) or ethanol are added to gasoline to achieve the required octane or to reduce certain emissions.

Other additives that may be added to gasoline include octane enhancers such as methylcyclopentadienyl manganese tricarbonyl (MMT), corrosion inhibitors, stabilizers, antioxidants, metal deactivators, demulsifiers and deposit control additives.