Originally Posted by sedate_driver

Let's assume your gear ratios are these:
Gear Type 6 Speed Automatic Final Drive 3.91:1 1st Gear Ratio 4.17:1 2nd Gear Ratio 2.34:1 3rd Gear Ratio 1.52:1 4th Gear Ratio 1.14:1 5th Gear Ratio 0.87:1 6th Gear Ratio 0.69:1
Then:
120 miles/hr x 5280 ft/mile x 12 in/ft x 1 hr/60 min = 126,720 in/min.
Assume tire diameter is 29 in.
Circumference is 29 x pi = 91.1 in.
Tire rotation speed is 126,720/91.1 = 1391
Rotational speed of drive shaft = 1391 x 3.91 = 5439
Gear ratio = 6200 rpm/5439 = 1.14
You are in 4th gear.