Kurt, I built a little Excel spreadsheet to plug the numbers into. The formula is:

Cruise RPM = (mph x Gear Ratio x 336 x OD Ratio)/Tire Diameter in inches.

The "336" is simply the required unit conversions melded into one value. Using your numbers shows that with a rear gear around 3.73 you'll be cranking 6469rpm at 160mph, and those tall tires are what lets you run the big gears. You won't run out of gear, but you'll have to have enough horsepower and torque at the top to push through the air and overcome friction at that speed.