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Gear inches is one way of measuring the bicycle gearing of a bicycle, so that different gears and different bicycles can be compared in a consistent manner.
Gear inches is an Imperial units measure corresponding to the diameter in Inch of the drive wheel of a penny-farthing bicycle with equivalent (direct-drive) gearing. A commonly used metric alternative is known as metres of development or rollout distance, which specifies how many a bicycle travels per revolution of the crank.
Typical gear ratios on bicycles range from very low or light gearing around 20 gear inches (1.6 metres per revolution), via medium gearing around 70 gear inches (5.6 m), to very high or heavy gearing around 125 gear inches (10 m). As in a car, low gearing is for going up hills and high gearing is for going fast.
This formula assumes that any hub gear is in direct drive. A further factor is needed for other gears (many online gear calculators have these factors built in for common hub gears).
For simplicity, 'gear inches' is normally rounded to the nearest whole number.
For example, suppose the drive wheel is actually 26 inches in diameter. If the front chainring and rear sprocket have equal numbers of teeth, one turn of the pedals produces exactly one turn of the drive wheel, just as if the pedals were directly driving the drive wheel. That combination of gears and wheel is said to be "26 gear inches." If the front chainring has 48 teeth and the rear sprocket has 24 teeth, then each turn of the pedals produces two turns of the rear wheel. This is equivalent to doubling the size of the drive wheel; that is, it is like a directly driven bicycle with a 52-inch wheel. That gear is said to be "52 gear inches."
A bicycle with a 26-inch wheel, a 48-tooth chainring, and a cassette with gears ranging from 11 to 34 teeth has a lowest gear of gear inches and a highest gear of gear inches.
For bicycles with 700c wheels, some cyclists quote gear inches based on a nominal wheel diameter of 27 inches, corresponding to the old British tire size of 27 x " (ETRTO 630). Strictly speaking, the rolling diameter of a 700c wheel may be significantly higher or lower than 27", depending on the tire size, e.g. nearly 27.5" for a 700x38 tire, or just over 26" for a 700x20 tire. This can be the source of some confusion when comparing gears unless it is clear whether gear inches have been calculated using the actual wheel size or a conventionalised 27".
One way to estimate wheel diameter is to add twice the nominal tyre cross-section to the rim diameter. For example, consider a tire, which has a nominal cross-section of 23 mm. 700c wheels have a rim diameter of 622 mm. Hence the wheel diameter is which is equal to 26.3 inches (rounded to 1 decimal place). 26 inch mountain bicycle wheels have a rim diameter of 559 mm. This ignores factors that contribute to the actual rolling radius of the tyre and rim together: the vertical deflection of the tire above the contact patch and the difference between the manufacturer's stated size and the actual tire radius when mounted and inflated.
Gear inches, along with cadence, can be converted to speed as follows:
or equivalently:
Thus gear inches and development differ by a factor of π times imperial-to-metric conversion. To convert from gear inches to metres of development, multiply by 0.08 (more precisely: 0.0254π).
| + 622/23 (700/23), combinations of number of teeth at the cassette and crank ! !40 !41 !42 !43 !44 !45 !46 !47 !48 !49 !50 !51 !52 !53 | ||||||||||||||
| 10 (Micro drive) | 8.39 | 8.60 | 8.81 | 9.02 | 9.23 | 9.44 | 9.65 | 9.86 | 10.07 | 10.28 | 10.49 | 10.70 | 10.91 | 11.12 |
| 11 | 7.64 | 7.83 | 8.02 | 8.21 | 8.39 | 8.58 | 8.77 | 8.96 | 9.15 | 9.34 | 9.55 | 9.74 | 9.93 | 10.12 |
| 12 | 6.99 | 7.18 | 7.35 | 7.51 | 7.70 | 7.87 | 8.04 | 8.23 | 8.39 | 8.56 | 8.75 | 8.92 | 9.09 | 9.28 |
| 13 | 6.46 | 6.61 | 6.78 | 6.95 | 7.09 | 7.26 | 7.43 | 7.60 | 7.74 | 7.91 | 8.08 | 8.23 | 8.39 | 8.56 |
| 14 | 6.00 | 6.15 | 6.30 | 6.44 | 6.59 | 6.74 | 6.90 | 7.05 | 7.20 | 7.35 | 7.49 | 7.64 | 7.79 | 7.95 |
| 15 | 5.60 | 5.73 | 5.88 | 6.02 | 6.15 | 6.30 | 6.44 | 6.57 | 6.72 | 6.86 | 6.99 | 7.14 | 7.28 | 7.41 |
| 16 | 5.25 | 5.37 | 5.52 | 5.65 | 5.77 | 5.90 | 6.04 | 6.17 | 6.30 | 6.42 | 6.57 | 6.69 | 6.82 | 6.95 |
| 17 | 4.93 | 5.06 | 5.18 | 5.31 | 5.44 | 5.56 | 5.69 | 5.79 | 5.92 | 6.04 | 6.17 | 6.30 | 6.42 | 6.55 |
| 18 | 4.66 | 4.78 | 4.89 | 5.02 | 5.12 | 5.25 | 5.37 | 5.48 | 5.60 | 5.71 | 5.83 | 5.94 | 6.06 | 6.17 |
| 19 | 4.43 | 4.53 | 4.64 | 4.74 | 4.87 | 4.97 | 5.08 | 5.18 | 5.31 | 5.41 | 5.52 | 5.62 | 5.75 | 5.86 |
| 20 | 4.20 | 4.30 | 4.41 | 4.51 | 4.62 | 4.72 | 4.83 | 4.93 | 5.04 | 5.14 | 5.25 | 5.35 | 5.46 | 5.56 |
| 21 | 3.99 | 4.09 | 4.20 | 4.30 | 4.41 | 4.49 | 4.60 | 4.70 | 4.81 | 4.89 | 4.99 | 5.10 | 5.20 | 5.29 |
Some national cycling federations limit rollout distances for younger riders.
For example, a common junior gearing until 2023 was 52 teeth on the largest ring at the crank, and 14 as the heaviest gear available in the rear wheel. Satisfy Junior Gearing Restrictions with Superior Shifting Coupled with a 700c wheel (622 mm diameter) and a 23 mm tire size (which approximately adds 2×23 mm to the wheel diameter), this gives an approximate diameter of 0.645 metres (depending on various factors such as air pressure, tire and rim), resulting in an estimated rollout distance of: π ⋅ 0.668 m ⋅ ~ 7.53 m
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