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Not to be pedantic, but you’re off by a factor of 100 :)
75kg * 1000g/kg * 0.15cal/g * 0.001cal/kcal = 11.25kcal
The .15cal/g is much closer to what I'm probably burning. According to my Garmin it was 540 calories which is 18kcal/km, and I weigh ~86kg which puts me at about .20cal/g, not much higher than Walton's number. And I was definitely not taking a casual pace, there was no coasting on that ride.
An interesting result of this is that if you're trying to lose weight, cycling isn't the most time-efficient way to burn calories!
Running is a much quicker way to burn calories. I hate running. :)
Certainly going to work by bike is better for weight itself then going by car.
"Certainly"? Not really. Depends entirely on your habits. If you get to work after your bike ride and eat a Snickers because it made you peckish, and you didn't do that when going by car, you've effectively done nothing for your weight.
It's true that exercise is good for you regardless. But it is orders of magnitude easier to ingest calories through food than it is to burn them through exercises.
If you go to work on bike regularly, your body don't even register it as something to triger you needing more food. If you are extra hungry after 30 min transport trip on bike, you are likely in very bad physical shape.
98% of diets end up failure in the long term, precisely because just skipping food does nit work in the long term. You end up miserable, tired, passive, anemic, underperfoming on the job.
Cycling is one of the very few sports (along with maybe swimming) that you can do for extreme amounts of time without acutely harming your body.
The issue that does affect people is that if you started with the bad diet, you probably don’t have the fitness to produce the required output that would overcome it.
If you manage to increase your fitness so that you sustain higher outputs however you absolutely can out-train your terrible diet!
Even with running - especially with running - you can easily burn enormous amounts of calories.
There are two problems with this saying.
First, nobody knows what a bad diet is in the context of this saying. How many calories are we talking? How poorly balanced is the diet? Let's take a basic example: assume the perfect diet for a sedentary person, well balanced, exactly the right amount of calories, etc. Then on top of it, this person eats a 200g pack of Haribo 4 times a week. Surely this makes that diet insanely bad, right? Well, not really, it's only about 500 excess kcal/day which would be very easily compensated if this person had an active lifestyle. So how insanely bad does the diet need to be until the argument actually works? This saying is usually directed at people who want to lose weight, surely nobody is trying to lose weight in good faith if they drink soda daily, eat donuts left and right, and some haribos to top it off.
The second problem, is that people are scarily sedentary and view what should be a completely normal amount of physical activity as impossible. So what upper bound are we putting on the "running" part of the saying?
Doing 1-2 hours of sport 4 times during the work week, plus one longer physical activity on the week end (e.g., half a day hike or bike ride) is a completely normal amount of sport.
If your goal is weight loss, and you have a choice between an extra hour of exercise, or eating one less Mars bar a week, skipping the Mars bar will be better for your weight loss.
If your goal is weight loss you should probably skip a mars bar every now and then, but I think you're underestimating what's possible after a bit of training and fitness improvement.
Counting kcal per hour of sport like that doesn't make any sense. 300kcal is 1h of brisk walking (non-sedentary people do it as part of their daily life, and probably don't count it toward their physical activity). It's also 1h of weight lifting, which most people would probably consider intensive sport. OTOH it's only 30min of jogging, which is a utterly trivial amount of sport for any healthy person. Those are rough approximations obviously.
After clicking the "reply" button of this post, I'm leaving for training with my sports club. 30min brisk walk each way to get there, and 1.5h of sport that is estimmated to burn 500kcal/h. Burning calories is trivial for active people.
Eating 3000 calories in one sitting is hard. Like, genuinely difficult, you're probably stuffing yourself uncomfortable and then in a food coma afterwards hard.
It reminds me of those silly TV shows with depressed 500lbs weirdos who just stuff their face all day. No-one normal is doing that. I mean christ it's only a bit less than half a kilo of straight mayonnaise.
This is all made more confusing by the fact that in most of the world "kcal" is just pronounced "calories" (ignoring the 'k' which is implied in this context), while in the US "Calories" (with a capital 'C') stands for kilo calories.
But back to your point, 0.15cal/g/km = 0.15kcal/kg/km indeed passes the intuition test. For a 100kg bike+rider package that would be 15kcal/km => 1500kcal for a 100km, which seems to be the correct order of magnitude. Of course this is an obvious oversimplification, but it gets the point its trying to make across.