nitrogen is a bigger molecule and will not seep through the pores of rubber. it does cost to get them filled but ir's suppose to be a 1 time fill unles you have a flat or change your tires. other problems - nitrogen filled tires are suppose to be marked with a blue cap but if you take your car in for service and they check the air, they may not know. not sure if its a problem or not. when i got my '05, it had blue caps. replaced the rear tires at discount tires and they have no nit. they said they check and adjust air on their tires at any time for free and guarantee immediate check so there's no need to have nitrogen.