Joe - I agree that slightly bigger tires (without lifting) probably adds only a minor amount to the frontal area drag. Since interference drag is a super complicated thing, I can't say how much effect that would be either. But, I DO think there will be more resistance to forward motion at a given speed, and not less.
The part that really concerns me is the loss of forward force on the road caused by translating the available axle torque through the longer moment arm (the tire's radius). It's easy to calculate this loss. Going from 31" tires to 33" tires, increases the moment arm by 1 inch, which is a 6.45% increase. This means the "motive" force (push) at the tire circumference is decreased by 6.45% for a given axle torque. That's a significant reduction in the factor that keeps you moving forward. It will have to be made up for somehow, and that somehow translates (in my opinion) to more fuel and air per charge in each cylinder (to get that 6+% back). The applied axle torque will have to be increased by 6.45%, to keep you at equilibrium (a given, constant speed).
- Jack
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