I filled up on Shell premium in Concordia, KS. I remember it was rated 91 octane. I always fill up my tank with as much gas as possible without overflowing it.
I took I-81 up to US 36 and headed east on it all the way to Hannibal, MO where it 'merges' into I-72. I was starting to get worried when I was getting close to Hannibal but saw there was still enough gas left to make it to Barry, IL where I filled back up with Shell premium. In Illinois, it is rated 92 octane.
Anyway my Mustang has a 15.7 gallon tank approximately according to the owner's manual. It took 15.2 gallons to fill her back up so...
385/15.2 = 25.3 MPG
Not bad at all for a 1998 Ford Mustang GT.
The drive had some speed, but we were going 75 - 80 MPH on average. I had to go to KS for my brother's wedding so I had a buddy riding with me (who owns a 1995 Ford Mustang GT I use to own) and we had more friends in a GMC Yukon following us and vice versa (we pass each other for fun and to relieve boredom, lol).
My friends in the Yukon thought I was crazy pushing it and my buddy in my car was cheering me on. My parents were in a 2004 Chevy S10 not far behind us so I figured if I ran out they could give me a ride to get some gas. Anyway it all it fine and I was shocked to get that kind of mileage out of that car. My buddy wanted to try it with his Mustang.
Now I usually use Shell premium when I take long road trips like that. The best I had ever got before that was 350 miles out of a tank and that was never speeding. This 385 mile tank did have speeding and I did get up to 100 or better a few times to pass my buddies in the Yukon back after they passed me.
I also recently went to Mobil 1 EP 5W-30 so that was more than likely a factor as well. Going 75-80 MPH was giving my car RPMs in the very low 2,000 range. There are lot of hills in KS, MO, and IL.
Lastly, I filled up in Hannibal, MO with Shell premium on the trip to KS. The car got 338 miles out of the tank before it had to be filled up again. I think the town we made it to was Belleville, KS. We were going faster on the trip there as opposed to the trip back home.
I guess it was good gas and good oil. I was just astounded it got 385 miles out of a tank of gas. My friend that was riding with me thought I was B-S'ing him about getting 350 out of a tank so 385 was a nice surprise for him and myself. My car is an automatic. His (which use to be mine) is a manual.