does having bigger rims and tires affect gas mileage at all?

like going from 17' runflats to 20' low profile tires..
if so, is there a big difference?

 

Tires are the largest part of drag on a car. Aero dynamically and rolling resistance wise.

So yup, you will get less MPG.

 

nope. assuming you keep the same rolling diameter. the 20" may give you better mpg due to less rolling friction of the smaller side wall flex. but it not worth the price nor the lack of handling you'll get form to 20"

 

For aero dynamic, are you referring to F1 racing? I don't think regular car has much to do between tires and aero dynamic

 

In your example, more than likely yes, but probably not enough for you to notice. Things like a wider tire and a possibly smaller tire diameter can have a small effect. The biggest issue will be the weight of the new wheel/tire combo. Depending on the wheel, you could easily increase the weight by 15 lbs or more per wheel. That will affect your milage as well as slow your acceleration and increase your braking distance. The tire will play a role as well. A sticky performance tire will offer more rolling resistance than a super-hard economy one.

I think if you keep the wheel weight to a minimum you should be okay.

 

Runflats are very heavy tires.

 

You have to also take into account weight of the wheels and tires. Lower weight = better gas mileage.