225/45-15 will have the larger tread contact patch.

03 G35 Coupe 6 M/T

 

Only 1 guess so far, but no that is not correct. In actuality, they have they same contact patch. The big clue for this is that they are running the same air pressure. The difference is that the 225/45 has a wider but shorter contact patch than the 205/50. There is actually an easy test you can do to prove this. If can wither barrow a wheel or 2 or get a pair of used tires to start. You just jack up a corner of a car and use it to swap wheels. You have 2 identical wheels with different tires and set both tires to 30psi static pressure. You jack it up the single corner on a flat surface. Coat the tire in water or paint(if you really want to see it well), put it on and set the car on the ground. Then jack it up and measure the width and length of the contact patch mark. Now jack it up and swap to the other wheel. Then do the same. Calculate the square inch area of the contact patch by multiplying the length by the width and they are exactly the same. tire composition can effect this, so a different belt or sidewall construction will change the size of the contact patch.

This is actually basic physics. The weight pressuring the wheel is constant(the weight of the vehicle), but there is a greater amount of air volume and a greater surface area to absorb the same amount of weight. The only place you gain is in peak cornering where weight is being shifted sideways away form the vehicle, over the plane of the tread. The only way you can gain in static contact patch is to reduce pressure. This is why they run really low pressures in drag slicks and radials. To do this when already overtiring a wheel means you have to increase the flexibility of the sidewall by reducing the stiffness(air pressure) against it and reduce the stability of the tire. This is where the wider rim is beneficial. The sidewall is stretched more, so it is stiffer and supported better. Lower tire pressure with a wider wheel/tire combination is still more stable since there is still a greater volume of air in the tire than the stock tire to support it. The 245/40's on my fronts actually fit a lot tighter than I was expecting them too, so they have a good amount of sidewall stiffness. I run 33 psi in the front, while 35 psi in the rear to get the additional traction in the front.

Note: if anyone who reads wants to learn more about suspension tuning, please check out the Evolution Driving School Dial-in

04' G35 Coupe(finally in the garage)
6-sp Diamond Graphite/Willow w/Premium

 

I e-mailed your question to an engineer friend of mine at Goodyear. His repsonse;
"The size of the patch will be close between the two, but the 225 will offer a wider and shorter patch for better lateral traction."

03 G35 Coupe 6 M/T