modifying my focus

Normally the difference in moment/lever arm (perpendicular distance) of the rim is offset by lower profile tires so that's really a null issue. It does tend to increase increase the rotating mass and by changing tire profiles you change the vehicles dynamics (way it moves under various loads). Basically don't change it too much either way. If the rotating mass increase the amount of power necessary to alter rotation is increased. IE it's harder to stop and harder to go. But as to what you were asking directly, yes making the wheel/tire combo larger does change the amount of force the tire puts to the road (torque is constantly applied what changes is the resultant at the ground). I'm not sure how to make this make sense... I'm trying to shorten an explanation and making it make no sense... So big tire=less force moving faster small tire=more force moving slower The rule of thumb is to stay with an inch or two of stock rim size. And get a tire to match the outer diameter to stock (You can find calculators online to do this). If not your speedometer will read wrong. Go bigger it reads slow (you're going faster than the speedo say=tickets)

Adam is on the right track. Changing the diameter of your wheel (rim) is fine, so long as you adjust the tire size and profile to keep the overall diameter of the Wheel+tire close to stock. This is a very common practice and it is called plus-sizing your wheels. Adam also makes a good point about rotating mass. Increasing the amount of mass the car has to rotate makes it more difficult to accelerate and decelerate. In addition, the wheel and tire package is unsprung mass. The weight of the wheel and tire package has a big influence on how quickly and smoothly the cars suspension can react to steering or road inputs. When plus sizing a car's wheels, it is important to realize there are some downsides. Depending on your wheel and tire selection, wheel/tire package overall mass is an issue, ride quality will deteriorate because the lower profile tire has less compliance, and th ecost of low profile larger diameter tires increase rapidly as you go to bigger diameters. For a Ford Focus, I would suggest you go no bigger than 17"

^ Much better explanation :-D