Kind of a catch 22 problem. Highway speed at low rpm needs RV cam or even less, RV cam means low power, not enough to go fast but MAYBE higher mileage. High HP cam means not enough power to get out of it's own tracks at low rpm, that's why long track roundy-rounders need to spin tires to get going. A little tough going from light to light with 7000 rpm launches. 4000 rpm = 167 mph, how fast do you want to go on the street, that's only about half or less of the rpm available to a really high hp engine. A cam needs to operate in it's designed range to get efficiency = mileage. Recommend going to "Spicer speed calculator" and play with some their toys.
Ron