Reuters: Luxury carmaker Porsche (PSHG_p.DE: Quote, Profile, Research) launched a legal challenge on Wednesday to a tax on gas-guzzling cars proposed by London mayor Ken Livingstone who is campaigning for a third term in office. Livingstone wants to raise the eight-pound ($15.90) daily levy on all cars driving in the city centre to 25 pounds for those with high fuel consumption, in a drive to cut CO2 emissions and fight global warming. Porsche called the plan unfair and disproportionate …