Insurance groups for cars are based on many factors, and while engine size and performance are contributory factors, other elements such as historical data showing how likely they are to be involved in an accident, repair costs, standard safety equipment and risk of theft all play a part, which is why two relatively similar vehicles can have different insurance groups. How much this will affect the premium will significantly depend on other criteria relating to the driver or drivers, usage, mileage, location and so on, so we would suggest using insurance comparison sites and switching between the vehicles to judge how this will affect you. It may actually be less than you think.