The math doesn't lie, more torque to the ground with a direct, as for Volvo versus Mack.....well I think you know where most of us stand there. The Volvo should pull better since it has lower overall gearing, the Mack may get better fuel mileage but I doubt it running 65. The issue here will be lugging or having to drop a gear constantly, without knowing tire size its hard to figure rpm @65mph but I wouldn't buy an OD truck with anything lower than a 3.36 with a 42" tall tire. I'm sure this will spark some debate but if you find somebody running 2.66 rears with an OD trans and ask them what their rpm is at 65 that should help you out. I would want my cruise rpm no lower than 1300-1400, any lower and any kind of rise and you'll need to drop a gear. I personally like a 1400-1600 rpm cruise speed depending on trans so you have the power to maintain road speed on rolling hills without changing gears or lugging constantly but I live in a hilly area.