Gear-Inch Chart
(27" or 700cc wheel, x-axis = front chain ring, y-axis =rear cog)

38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

11

93"

95

98

100

103

105

108

110

112

115

117

120

122

125

127

130

132

135"

12

85

87

90

92

94

96

99

101

103

105

108

110

112

114

116

119

121

123

13

78

81

83

85

87

89

91

93

95

97

99

101

103

105

108

110

112

114

14

73

75

77

79

81

82

84

86

88

90

92

94

96

98

100

102

104

106

15

68

70

72

73

75

77

79

81

82

84

86

88

90

91

93

95

97

99

16

64

65

67

69

70

72

74

75

77

79

81

82

84

86

87

89

91

92

17

60

61

63

65

66

68

69

71

73

74

76

77

79

81

82

84

85

87

18

57

58

60

61

63

64

66

67

69

70

72

73

75

76

78

79

81

82

19

54

55

56

58

59

61

62

63

65

66

68

69

71

72

73

75

76

78

20

51

52

54

55

56

58

59

60

62

63

64

66

67

68

70

71

72

74

21

48

50

51

52

54

55

56

57

59

60

61

63

64

65

66

68

69

70

22

46

47

49

50

51

52

54

55

56

57

58

60

61

62

63

65

66

67

23

44

45

46

48

49

50

51

52

54

55

56

57

58

59

61

62

63

64

24

42

43

45

46

47

48

49

50

51

52

54

55

56

57

58

59

60

61

25

41

42

43

44

45

46

47

48

49

50

51

52

54

55

56

57

58

59

26

39

40

41

42

43

44

45

46

47

48

49

50

51

52

54

55

56

57

27

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

28

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

29

35"

36

37

38

39

40

40

41

42

43

44

45

46

47

48

49

50

51"

Notes about Gear Inches

Gear ratio is calculated as the size (number of teeth) of the front chain-ring divided by the size (number of teeth) of the rear cog. Gear inches are calculated as the Gear Ratio multiplied by the diameter of the wheel, 27 inches for a road bike (roughly equal to 700cc tires - tire size also is a factor, but this will be accurate to within a few inches).

Recommended low-gear ranges
based on rider strength

From "Complete Guide to Bicycle Maintenance and Repair" By Jim Langley:

Type of riding Terrain Gear Inches
Loaded Touring Steep hills 20-27
Medium hills 32-42
Sport Touring Steep hills 27-37
Medium hills 32-42
Flat 37-47
Road Racing Steep hills 47-60
Flat to rolling 57-66
Mtn bike Steep hills 17-20
Technical trails 17-20
Dirt roads 20-27
Fixed Gear* Track 80-90
Road 60-70

* fixed-gear data thrown in by veloroutes.org