Fuel compositions

A method of increasing the power output of a spark ignition internal combustion engine wherein the method comprises adding a poly butene and a tetraalkylethane compound to a gasoline base fuel to produce a gasoline fuel composition, wherein the polybutene polymer is added at a level from 1000 ppm to 5000 ppm by weight of the gasoline fuel composition, and wherein the tetraalkylethane compound is added at a level from 200 ppm to 5000 ppm by weight of the gasoline fuel composition, wherein the poly butene has a number average molecular weight in the range from 200 to 10,000 g/mol and preferably wherein the poly butene is a high reactivity polybutene wherein greater than 30% of the polymer molecules in the polyisobutylene polymer have a terminal vinylidene group and wherein the tetraalkylethane compound has the formula (I):

• wherein Ar represents an aryl group and each X is independently selected from a hydrogen atom, substituted or unsubstituted, straight chain or branched C1-C12 alkyl group, (CH2)nOH or (CH2)nNH2, wherein n is in the range of 1 to 9, provided that at least one of the X groups in each CX3 group is a hydrogen atom, and combusting the fuel composition in the spark ignition internal combustion engine.