[HummerJim]

You're right, the manual does say premium and I've found that by airing up my tires to 41 psi and running a constant 65 mph I got 13.3 mpg going out highway 70 yesterday. It seems the low tire pressure made a BIG difference in my mileage. When they delivered mine they said I could use midgrade 89, but the transmission doesn't hunt for gears with the premium gas on grades, so I'm sticking with premium for the engine's sake as well as the better mileage. I must honestly say though, I didn't hear any knocking or pinging with regular 87, just more gear shifting in the tranny. It definitely accelerates better with premium!