From your lips to God's ears. I have a 2001 Corvette that gets 32 mpg @60 mph in 6th gear. It also does 0-60 in 4.7 seconds when I'm feeling frisky.
Manual Trans C4? 'Vette's are the poster child for performance and economy. There's no reason a Suburban shouldn't be able to do at least 25 mpg at 60mph. Looks like I have to (re)build my own.