Highway mileage should be near 30, so I'm betting if you drove it highway, you could take it to 35 w/ some careful driving.
My Z28 before mods was getting 28 and 29mpg highway, now w/ tuning, full exhaust, gears, and pushing 18" wheels w/ 315 rubber around, it's lucky to get over 20 highway.
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John
'09 Saturn Aura 2.4L
'94 Chevy Camaro Z28 (5.7L 6sp)
'96 Chevy C1500 (5.0L 5sp)
'08 Kawasaki Vulcan 900 Custom
'01 KTM Duke 2