Mine is about 25mpg's on the hwy and about 20 in town. I'm happy with that and she runs like a top.
C.
The only real distance driving I've done so far is when I bought it (1300 miles).
I got best of 32 mpg out of it on the highway at 75 driving from N.M. to Mn.
Question - why are you putting premium in your Vette? If you have the L98 as you said, it's unmodified then it should have 9.5-1 compression ratio. That should run on 87 octane regular just fine. You're not getting any benefit by running premium and actually may be hurting things because you want higher compression or something like a blower to take advantages of the characteristics of premium.
I admit that gasohol is hard on rubber parts but even if you have to fuel with non-alcohol gas for collector cars and small engines, it's still cheaper here than paying for premium. Having said all that, with a 2.59 gear ratio, I would expect yours to do very well on the highway using cruise. Now all this depends also on driving style, if you're a stoplight racer it will kill gas mileage every time.
Mine has the same compression ratio and I've been burning 87 octane in it since I bought it last year except when I put it into winter storage last fall. Then I used non-alcohol gas. It drives just fine on 10-15% gasohol.
Just my 2 cents.
1991 ZR-1 # 118 SW Headers/Corsa-3:73 gears-Haibeck-510HP/350 1993 ZR-1 # 179 Yellow/ White- Haibeck tune, Flowmasters
Question - why are you putting premium in your Vette? If you have the L98 as you said, it's unmodified then it should have 9.5-1 compression ratio. That should run on 87 octane regular just fine. You're not getting any benefit by running premium