Justin is not correct. It is true that the diameter of the larger circle is 2 inches more than the smaller circle diameter.
Use formula pi D or 3.14xDiameter
Large gear:
3.14(4)
=12.56 in
smaller gear
3.14(2)
=6.28
actually, the larger gear in circumference is 6.28 more than the smaller gear. Also, 12.56/2=6.28 ...so, the larger gear circumference is 2 times the diameter of the smaller gear.
12.56-6.28=6.28