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.
2013 Jeep JKU Sport S white auto 3.73, lifted on 35's
2003 Jeep TJ Silver 4.0L, Auto, 5.13 G2 D44's on 35's
1998 Jeep TJ Sahara Black 4.0L 5-Speed, Replacing the frame
1997 "GI" Jeep TJ OD Green 2.5L, 5-Speed, lifted on 33's