During the cold winter months when snow and ice are on the ground, it's too dangerous to ride a bike, so many cyclists run instead. The standard comparison is that one mile of running equals four miles of cycling, but that's lousy science. Although running requires the same amount of energy per mile at any speed (110 calories per mile), riding is affected by wind resistance so the faster you ride, the more energy you use. So you have to compare running and cycling at different cycling speeds.

Dr. Edward Coyle of The University of Texas in Austin determined average values of oxygen consumption by cyclists to develop a table to estimate the approximate caloric equivalence between running and cycling. He found that if you ride 20 miles at 15 mph, you burn 620 calories (20 miles X 31 calories per mile = 620 calories). Take the 620 calories and divide them by 110 calories per mile for running and you get 5.63 miles to burn the same number of calories. So riding a bicycle 20 miles at 15 miles per hour is equal to running 5.6 miles at any speed.

Coyle's derived conversion figures are for an average-size adult (approximately 155 pounds). A larger cyclist would divide by a slightly higher number; a smaller cyclist, by a slightly lower one. Wind and hills are not accounted for in the table; nor is drafting, which can reduce your energy expenditure by up to one-third.

The number of miles ridden divided by the conversion factor for the speed of riding equals the number of miles running to use the same amount of energy. Here's the conversion table:

MPH:Calories per mile:Conversion factor
10: 26: 4.2
15: 31: 3.5
20: 38: 2.9
25: 47: 2.3
30: 59: 1.9

How to use the table: For riding 20 miles at 10 miles per hour, divide 20 miles by the conversion factor of 4.2 to get 4.8 miles running. For riding at 20 miles at 20 miles per hour, divide 20 miles distance by 2.9 conversion factor to get 6.9 miles running. For riding 20 miles at 25 miles per hour, divide 20 miles by 2.3 to get 8.7 miles running. For riding 20 miles at 30 miles per hour, divide 20 miles by 1.9 to get 10.5 miles running.