My YJ with no top and no carpet was much quiter than my Jeep J-10 Truck that I had for years before my wrangler.
The gas mileage in my YJ is about 20 city/highway. She doesn't like to go over 60 MPH on the Highway and when in town, she typically goes 5 under the speed limit, typically so I can just enjoy the drive. (for point of reference, my TJ gets 13-15 MPG for the same type of driving)
I had friends that had CJs so I knew what I was getting into. I was a bit disappointed when my Jeep didn't wander all over the lane like my friends - I thought that came standard
So if your friends don't have one - I agree with renting one and checking it out for yourself. I used to be a speed demon because I just wanted to get to my destination. Now when I'm driving, I do it slowly and hope I never reach my destination.