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'Low gear' gives the shortest distance travelled for one turn of a crank.
This also equates to effort - a high gear is harder to push than a low gear.
The reason big cogs give you low gears on the rear but high gears on the front is that front and rear are doing different jobs.
On the front, the chainring, the cog is pulling the chain - the larger the cog, the more chain it will pull with one turn of the crank.
On the rear, the freewheel, the cog is being turned BY the chain - the smaller the cog, the more revolutions will be generated by the same length of chain.
Hence, a large chainring will pull a lot of chain through which will turn a small freewheel a lot of times, thus giving you a higher gear.
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The most common way to express a final ratio is in "gear inches", which is the equivalent wheel size you'd need to produce that ratio if the cranks were turning the wheel directly.
You can work this out easily, by counting teeth on the front and rear, dividing the two to get a drive ratio, and multiplying by the wheel size.
e.g, really high gear on 26" wheels:
52t front 11t rear, 52/11 = 4.7, 4.7 x 26" = 123 gear inches.
low MTB gear:
24t front 34t back, 24/34 = 0.7, 0.7 * 26" = 18.3 gear inches.
Big number of gear inches is a high gear. Small is low gear. See, easy.
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