360 magnum horsepower guess

A 3.55 gear would have you running at 2900 rpm at 61 mph accounting for 1/2 inch of tire growth at speed. 3.91 gears will have you at 3200 rpm at 61 mph. 3.23 gears will have you at 62 @ 2700. I would look into getting a set of the largest diameter highway tires that will reasonably fit on some 15 inch wintertime rims to run during the highway trips.