Here's an answer with some logic and a little anecdote...

I would think the max efficiency is right before the supercharger kicks in at 1600 RPM. Which combined with the squared velocity effect of drag vs. MPH, makes the speed of 40-45 MPH the most efficient speed. Here's a little data: I just moved from San Antonio to Niagara Falls. My commute changed from 25-35 MPH neighborhoods to 40-45 parkway. My mileage increased dramatically--more so than entire tankfuls on the freeway. This would comfirm my hunch of just under 1600 RPM.

Having said all that, a momentus occasion: I got 300 miles on a tankful for the very first time! Woo-hoo!

Hope that helps.

Shawn