I'd look critically at the present tires. Any excuse to go to a larger tire, or not?
The other half of the equation, from power delivered to the tires contact patch, is what it takes to punch a hole in the air. ....and close it again. How many feet of boat tail could you tolerate?
For comparison -- in the 1980s I put a fresh rebuild in my 1961 VW Type II panel van (1600, dual Solexes, 4 into 1 extractor exhaust) and drove 60 miles through the Coast Range at 55MPH. It took exactly 2 gallons, so suggesting 30 MPG. It cruised at 75 thereafter (gas pressurized shocks and quartz-halogen headlights).
Back in the last decade, I turned it into money to put into the Superbeetle.
