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.
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