I think doober wants a landspeed car, HAHA!
My favorite combination was in my 96 GMC JIMMY. 4.3L/4l60E/3.08/30 inch tall tires. After a few performance tweaks, I was getting 28 mpg out of it consistently, and it pulled pretty hard for what it was. Later on, I changed it to 3.42 rear. Didn't feel all that much peppier, but it would pull sideways on the 1-2 shift at full throttle. Mileage dropped down big time, never topped 21 after the 3.42s.
I sold a 87 C10 with a TBI 305/700r4/2.73/28 inch tires. It as well would get some 26 MPG on the hwy, but it liked to eat transmissions (you didnt worry about MPG, you worried about MPT- miles per transmission) . It was an absolute dog.
I don't know what the final numbers work out to, but my current pickup is TBI 350/700r4/3.42/35.5 inch tall tires. 40MPH sounds like it is idling. I was getting 19 MPG with the stock 30 inch tires and no topper.
Before you say anything, I know how to figure mileage, and I don't inflate numbers. I am that weird guy that can drive your car and get way better mileage than you most of the time. I had a guy bring is GPS and computer in my 87 C10 to verify the speedo, and filled the tanks at the same exact pump. I have also had a hand full of vehicles that never got better then 6 MPG.