My humpster has a 3.15 ratio with a 27.5" tall tire. It's happy at 60, 65.
It'll actually get mid-teens for mileage if you keep it around that speed. Running 70, 75 will get you there quicker but will slurp up the fuel in no time.

My Gremlin has a 3.08 rear and no overdrive (yet!). The little half-dozen is generally buzzin' around 2,500 out on the highway around 60. That car is the most fun to drive. At 60, it's still got plenty of go left. If you need to pass, just leave it in 3rd, and walk on it. It just wakes up and goes around whatever you wanted to pass.

Love that little six!