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One reason I kept the math fairly simple was that I figured I wouldn't be able to really sort out the RR contribution, and I assumed it would be nearly constant between 45 and 55 MPH. It rises a bit of course, but I think that the assumptions get us close to the answer. As 3-Wheeler pointed out, the required Cd reduction is a bit overstated because the RR component isn't a cubic relationship to speed.
Having done the math, one could come to a quicker intuitive feel for the problem of getting the lean burn threshold moved up. The MAJOR drag at these speeds is wind and since it goes up with the cube of speed, then getting Cd down enough to make a major threshold difference is going be very hard. Of course, this is all Insight specific. Other cars with other engines may perform differently.
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