Nekko, first off Welcome to CAC.
I don't know what you are thinking is "not great mileage". Fuel economy? But I've owned 4 corvettes over the years and for the performance I received from them they all gave "pretty good mileage". Beings you talking about C4s and C5s, I'll limit my discussion to them. If you talking about fuel economy like a Prius...no it won't get 40+ mpg.
My 94 (C4) had the LT1 5.7 rated at 300hp. When my wife and I took it on trips (2500-3500 miles), 1) we had plenty of room for my golf clubs and the luggage we required for a two week trip...we did that on several occasions. With the C4 judicious planning of the type of luggage you used is required if you are taking golf clubs. We used smaller soft sided bags rather than largehard sided cases. 2) On these trips, we would regularly see 24-27 mpg while running down the road at (or maybe a little above
) the speed limits. The seats (mine were the sport seats) were/are very comfortable for those long journeys. the C4 does have a high sill as you get into the car...really you step down into the C4.
The 2001 (C5) had the LS1 5.7 rated at 350hp. We took this car on several longer trips as well, one trip was 4800 miles in 13 days. Again 1) we had even more room for luggage in the 01 as compared to the 94. Could have carried 2 sets of clubs and all our luggage, but my wife doesn't play, so she just utilized the space by using different bags. The C5 has a large compartment in the back under the floor that we kept things that we didn't need all the time (or weren't sure we would use at all, but would be handy if needed). Actually we have a Cadillac CTS, and the C5 had more luggage space for clubs and luggage than it does. 2) The fuel economy on the 01 was about the same...maybe a little better than the 94. On our trip to California (the 4800 mile round trip) we averaged almost 27 mpg (26.7 IIRC). We went out on I40 and much of that portion of the trip was running 80-90 mph (just to keep up with traffic and not get our butt run over by the big trucks :L ) The return trip was by way of I80, and most of that part of the trip was run at 80-85 mph. The seats (again I had the sport seats) are very comfortable for those long trips and the interior is more spacious and entry and egress are much easier in the C5 than the C4.
So as you see, Fuel economy isn't all that bad. And personally, you don't buy a Corvette, necessarily for fuel economy. And I don't believe you'll ever catch me in a Prius.......
If I were you, I'd probably be looking for a nice C5. I don't know how big you are, but the C5 has more spacious interior, easier to get in and out of and has more power (in stock format). The C4 is more cramped interior, especially in the foot wheels. It feels more like a 'sports car". Both handle great, look great and are both great "bang for the buck" sports cars.
If you have any specific questions, don't hesitate to ask. And good luck with your search and purchase of America's true sports car.