Many of us experience the opposite of what you posted--we get less MPG with the cruise control while driving on the interstate.

2005 AWD Escape Hybrid
Best Interstate tank trip MPG 39.02 (scangauge II) for 402 miles on I-70, 10.3 gallons used over mostly flat terrain.

Best Interstate tank trip E30 MPG 34.6 for 271 miles along I-80 in Indiana and Ohio.

Best multiple road tank trip E30 MPG 36.2 for 202 miles in Southwestern Pa.