# Speed and the sizes of tires/gears

Does the speed of a normal mountain bike vary according to the diameter of the wheels? If it does, will a larger diameter increase the speed or decrease it?

Similarly, if the speed varies according to the size of the gears, will a larger gear (if possible) increase the speed? What are the demerits/disadvantages of such features?

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For a single gear ratio turning at a constant cadence (revolutions per minute of one pedal), a larger wheel diameter will have traveled farther/faster. A larger wheel would increase top end speed, a smaller wheel decreasing it.

Gear ratios are gear ratios. For a given chainring tooth count moving at a given cadence, the smaller the gear in the rear the faster the wheel will turn. The larger the gear in the rear, the slower.

Basically, if you took a road bike that was designed for 700c wheels, and put 20" bmx wheels on it, it would be slower at a given cadence in all gears. The opposite would be true if you could increase the wheel size.

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Note that gearing is often described in "gear inches". This odd term is a little hard to explain unless you understand that it originates from the old "penny-farthing" bicycles with the small rear wheel and the big front wheel, with the pedals directly attached to the front wheel.

With such a bike, the larger the front wheel diameter, the faster you traveled for a given pedaling "cadence" (but the harder it was to pedal). So the front wheel diameter determined the "gear ratio".

With a modern chain-driven bike a given combination of front and rear sprockets combined with the diameter of the rear (drive) wheel will produce a gear ratio that is expressed in "gear inches". Basically, that "gear inch" number is the same as the diameter of the penny-farthing front wheel that would travel the same distance with one turn of the pedals. (And the larger the "gear inch" number the faster you go at a given pedal cadence, but the harder it is to pedal.)

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