.....or you can just divide the chainring tooth count by the cog tooth count to get a gear ratio. Let's say you're looking at the difference between a 34T chainring on a compact crank set and a 39T chainring on a standard crank set, both used with cassette whose largest cog is 25T. The first combo gives a 1.36 gear ratio, and the second combo gives a 1.56 gear ratio. A lower gear ratio is easier, and in this case, the compact option is [(1.56-1.36)/1.56]*100=12.8% lower. An example of a quasi-typical low gear with a compact crank set is a 34 x 28 combo, which yields a 1.21 gear ratio, 22% and 11% different respectively from the two aforementioned combos. The gear ratio tells you how many times the rear wheel turns with every revolution of the crank set. Some people are more comfortable with developed gear ratios, which are just the gear ratios multiplied by the rear tire circumference. Developed gear ratios tell you how far one revolution of the crank set will move you down or up the road. It doesn't matter whether you use developed ratios or just the plain jane gear ratio: the percent difference is the same either way. For me, it's natural just to think in terms of the gear ratio because at any instantaneous moment, the difference in force required to turn the crank set is the difference in gear ratios. A 12% lower gear requires 12% less force to turn the cranks for a given road condition.