Originally Posted by FiftySix

True.



That red bike is 66" with 38 tooth front / 16 tooth rear sprockets and 700x42 tires. 700 tires usually have a larger diameter than 26 inch tires, which will increase the gear inches number if the sprocket ratio stays the same.

Regarding the 70" I mentioned, my wife's 7 speed Schwinn and my 8 speed Norco both pull about 70" in 5th and 6th gears respectively. Those two bikes have front sprockets (aka chainrings) of 46 teeth and 42 teeth, respectively.

If you haven't seen it already, below is a short version of where the term Gear Inches came from.

"When the high wheeler or penny-farthing was the "ordinary" bicycle form, the comparative diameter in inches of the driven wheel was an indication of relative speed and effort. A 60-inch wheel propelled a bicycle faster than a 50-inch wheel when both were cranked at the same cadence. The technology of the high wheeler imposed a natural limit—a 60-inch wheel was about the maximum size that could be straddled by ordinary sized legs. When "safeties" replaced "ordinaries," chains and sprockets allowed small wheels to be turned faster than the pedal cranks. As result, a 28-inch wheel could be made to move a bicycle at the same speed as a 60-inch wheel. Such a bicycle was then said to be geared at 60 gear inches and pedalled similar to an ordinary with a 60-inch wheel. Thus on a bicycle geared at 72 gear inches one revolution of the pedals advances the bicycle the distance that a 72-inch wheel would in one revolution." - https://en.wikipedia.org/wiki/Gear_inches