A number of states produce calorie-charts for work/exercise done. And Wisconsin produces some data and it matches what Physics would tell us. Cycling one mile and the energy consumed doing it is basically a function of linear work (i.e. - here comes the Math: Force x Distance). Force is equal to mass x acceleration where mass is the mass of the bike and rider and gear, and acceleration is what is necessary to overcome the sum of wind and rolling resistance and other forms of dissipative friction.

What this tells you is that for distance being the same (1mile), and rolling/wind resistance just a slightly higher number for bigger riders on less race-oriented bikes, the major factor in how many calories get burned per mile is mostly body mass and maybe a little due to frontal area and higher rolling resistance. Bigger person, more calories spent per mile. So for someone 130lbs on a 20lb bike, it's around 35 calories/mile (or 20 kcal/km for those not in the US). For someone 155 lbs plus 20+ lb bike, it's 45 cals/mile, and someone 190 lbs, it's a bit over 55 cals/mile - according to Wisconsin. See http://www.livestrong.com/article/13...king-one-mile/. And the variance is relatively small because Work has no time-component in it's factor. You could ride slow or fast. At higher speeds, you work for shorter time. Work may increase slightly at higher speeds because wind resistance increases as the cube of relative windspeed. But for bigger riders, the frontal surface area to mass ratio increases more slowly, to counter-act that cube-power increase.

So someone who's like 300lbs on a bomb-proof tank bike, may expect to burn 80 calories/mile. So if someone rode 20 miles in 1hr 20 min. Then 80cals/mile x 20 miles = 1600 cals. Yes. I used to ride 20 miles a day in college. I ate 6000 calories a day. I was 255lbs and built like a linebacker with just 7% body fat. It takes a body mostly muscle about 35 cals/kg of wt. to just survive and maintain weight. That means, if you weight 116 kg, just to support that weight, you must eat 116kg x 35 cals/kg = 4000+ cals per day! Yes! You can eat two slices of Costco pizza AND a chicken bake and not gain weight if you are mostly muscle and big. Then add the fact that you ride 20 miles per day, with 3000+ ft of elevation gain. That adds another roughly 85 cals/mile x 20 miles = 1700 cals. And we getting close to needing to eat around 6000 calories per day just to maintain weight.

However, here's the problem - if you get sedentary and put on fat, the body shifts and now you only burn 30 cals/kg of body weight. That's a 14% drop in calories needed. So if you stop riding, get fat, then you must cut 14% off the 4000kg maintainance, plus you aren't burning 2000 calories a day in hill work on the bike. So you gotta cut something like 2500 calories per day out of the diet. That's brutal for folks who love to eat. If you don't - then it's easy to put on even more fat, over load the liver, over load pancreas, increase chances for diabetes, etc.