are aftermarket rims and tires beenficial for a car?

After market Rims don't really have any effect on the car. The only exception may be that if you get a heavy iron rim, maybe the extra weight could lower your miles to the gallon ratio. The real difference is in the tire width. Wider tires gives better traction, but thinner tires give a little better gas mileage. More tread more traction, less tread more miles to the gallon.

The thinner tires shouldn't give you any problems in drivability as long as your not trying to take corners or make turns at speed over the posted limit. a couple of inches larger rim diameter should hurt you either as long as your not taking pot holes and speed bumbs too fast. The larger diameter Rim should make up for the traction loss of a thinner tire in turning, because it will reduce the roll of the tire while turning.

Only if you consider being held up at the gas station by someone who wants to steal your rims a benefit. The car will handle and ride WORSE and may even vibrate. Tires will be much more expensive and they will wear out faster. I had a 2003 Infiniti I 35 that I couldn't even drive over 50mph it shook so bad with 20s on it.

well you need to say fairly close to the stock over all diameter of the stock wheel and tire combo.To large and it will through the gas mileage and speedometer off. With the speedometer off it may cause to get speeding tickets .Also the larger wheel the thinner the side wall will have to be on the tire . that will change the ride on the car. It will be a lot harder ride . So do your home work before you change the wheels and tires.

There are no benefits that would convince your parents to buy you aftermarket rims. I mean you can find rims that will weigh less than stock, and are wider, so you put more rubber on the ground and corner better, but i don't think that "Hey dad, can you buy me those rims and tires so i can street race people" line will work.

And handling wise, it goes down after you go above 18 in diameter.