My 97 has a 2.5L, 5-speed, 4.10 gears with 33" tires and I can cruise at 70 MPH as long as I don't hit hills. When I hit hills on the highway I downshift to 4th as my speed starts to drop but it never drops below 55 MPH. You should be able to maintain 55 MPH with your 31" tires.
2003 Jeep TJ Silver 4.0L, Auto, ARB locked 5.13 G2 D44's on 35's
1998 "GI Jeep2" TJ Desert Tan 4.0L 5-Speed, lifted on 33's (new frame)
1997 "GI Jeep" TJ OD Green 2.5L, 5-Speed, lifted on 33's