Fuel composition

Typical Composition of Gasoline (or Petroleum) throughout the world are as follows.
Heptane and Iso-octane...30-50%
Cyclopentane...20-30%
Ethyl benzene...20-30%.
Together with smaller amounts of other Alkane and Cyclic hydrocarbon compounds.
The Iso-octane has an Octane number of 100 while Heptane' Octane No. is Zero.

The above percentages are varied to produce the various 'Octane Ratings' of petrol (gasoline) according to their 'Grade'. (Modern petrol production is now blended to provide the required 'Anti-knock' qualities that used to be produced by the addition of Tetra-ethyl and Tetra-methyl Lead additives.

As in, 'Regular'...Octane rating of 87, 'Midgrade' is 89, and 'Premium' is 91 or 92.
(In mountainous regions Octane ratings are lower due to the effects of thin air on internal combustion.)