With regards to your extra wieght of the tire very good point also, I wasnt really concidering the tire weight. Does any one know the differences in weight of stock tires vs. 225s?

I think the differences in tire weight are going to be much less than that of wheel weight, rubber has a lighter weigh pet volume than metal. Given a wheel weights 25lbs say I can't imagine that a tire weighs more than say 10lbs at most. If I'm wrong on this someone please correct me. In which case the tire will have more of an effect than I thought. And I think you only need to multiply the extra weight by 2 since its a FWD car and not an AWD car. We can concentrate on the wheels that the engine is directly turning. I think we can look at the extra friction and drag the rear tires create by being wider. This is the reason drag racing cars have bicycle tires up front. But I think you're right we need to look at the outer edge weight.

Let's concider the weight distribution:
Assuming same width tires say 225s one on 14" wheels one on 18: wheels. on the 14" the concentration of mass is closer to the center allowing the wheel to spin more easily. On the 18" wheels the tire and weigh of the rim are concetrated around the outer edge of the rotation mass. Which should make it harder to turn. I still think that the rim has a larger effect, based on what I think the weigh of the wheels vs. the tires is. If someone knows the weights of the tires alone I may change my thoery.

Also the smaller rim with larger tires is going to has it's mass closer to the center wich would in part offset the larger rotating mass of the wider tire. If this is done on a 18" rim you are only adding to the outer mass, or at the very least not taking much away from it. Let me know what you think.

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