Paul -- the combo of light weight rims/tires ads up to the micro-seconds if/when pushing it to the limits on timed competition runs.. the driver still has the final limits pending skill/experience and overall car setup. If/when possible and the budget allows... always go for the lightest rims... light weight rims typically increase in cost... there's some serious choices available if someone really wants light weight rims... The most serious competition racers will spend those big bucks ... as it always comes to the overall package... everything continues to add up towards that package.