I went for a 32 mile ride today. Strava tells me I used 1321KJ of energy.

Then I filled up my car with petrol. Between my last fill-up and this one, I used 44L of petrol to drive 400 miles. That’s 9.09 miles per litre.

The internet tells me that a litre of petrol contains 32MJ of energy.

So had I driven my ride today, I would have burned 3.52 litres of petrol. That is, 112,640,000 Joules of energy.

Summary:

Cycling 32 miles takes 1,321,000 Joules.
Driving 32 miles takes 112,640,000 Joules.

Put another way, it takes 85 times less energy to propel me and a bike than it does to propel me and my car.

Now:

My car, with me in it, weighs 1117Kg. My bike, with me on it, weighs 88Kg.

So:

Riding my bike round that route used 15,011 Joules per kilogram.
Driving my car the same distance would have used 100841 Joules per kilogram.

Thus: Even if my car had the same mass as my bike, it would still be 6.72 times more energy efficient to cycle rather than taking the car.

NB: Even though I live in a hilly area, my average car journey is not as hilly as my ride was today (3171ft climbed in 32 miles). Factoring in vertical distance climbed by my car would therefore increase the “energy gap” between car use and bike use.

Ahhhh. I enjoyed that. Right. Pull my maths apart…
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NOW

Heat and friction. Engines are really bad at converting all that energy to forward motion, and each moving part costs energy in friction.

Yep. Cars are really, really inefficient.

I think a modern engine is about 25% thermodynamic efficient. So, you also have the large drivetrain losses, and of course the increase in air resistance from the (probably) higher speed of the car.

One thing to note…..

Your car may be 85 times less efficient (if you did your maths right)

but in terms of energy density per GBP… Petrol is 28 times better than food. Which brings the numbers a bit closer together.

I worked this out using the current price and calorific values of both petrol and Big Macs.

Also most people don’t just eat high energy and low cost Big Macs all the time – with a normal healthy diet it may be that fueling your car is in fact cheaper than fueling your body.

Cycling clearly has health and sustainability if not financial benefits and I am not advocating excessive use of the car

Just saying

This thread needs some graphs and charts

You are assuming your digestive and propulsion system is 100% efficient. Which it isn’t by a huge margin!

(ie to get 1321KJ of energy to your bike, you will have had to eat food with a much much higher energy value!)

[sadgeek]Is your 1321 kJ work done cycling, or total energy expended?

Say your power output during the ride was 200 W, and say you took 2 hours (7200 seconds), then total work done would be 1440 kJ, not that far from your measured 1321 kJ. I therefore suspect it is the useful work done.

You are physiologically about 20-25% efficient which suggests that your total energy consumption is probably about 5-6000 kJ. It makes the difference between you and the car a bit smaller, but doesn’t invalidate your point. [/sadgeek]

From Strava:

Energy Output measures the amount of work you’ve done during a ride, expressed in kilojoules (KJ). It is a factor of how much you’re pedaling, how fast you’re pedaling and how much force you’re exerting on the pedals (measured in W). Power output is most accurately taken from a power meter, but if you don’t have a power meter we give a rough approximation through our power estimator.

I’m not sure this helps…

Oh yes. Didn’t think of human inefficiency…but I suppose that makes it even more expensive to cycle

How much energy would you have used to do that cycle ride if you’d gone at the same speed you drive at?

Though as already pointed out, the big flaw is that you’re using energy in for the car and energy out for the bike.

How about adding a comparison with running and/or walking?

But yeah, you would have to go some way to make cycling less energy efficient than driving a brum car…

Was discussing this on a long climb at the weekend and remembered this:

“The bicycle is the perfect transducer to match man’s metabolic energy to the impedance of locomotion. Equipped with this tool, man outstrips the efficiency of not only all machines but all other animals as well.”

Ivan Illich

As above, Strava is only reporting the energy used by the bike. You’ll have produced a lot of heat as well, plus energy lost in inefficiency in the bike. 1321kJ is about 316 kcal, which is a lot less food than you’d expect to eat for a 32m ride, so not really comparable with the input energy to the car. Some of the remaining difference is covered by the air resistance of the car compared to the bike, certainly being bigger and probably faster. It would be interesting to see how much fuel a motorbike used, at the same speed as cycling.

But, interesting all the same, and the bike is still more efficient.

I was talking about this at work on Friday.

It’s already been pointed out that the strava metric is output rather than input.Taking this into account, we reckoned that “mpg” for walking was roughly equal to a car, and cycling was about one order of magnitude better.

If you’re eating especially fossil fuel intensive food, it seemed possible that it would be more environmentally friendly to drive than walk.

interesting article on Road CC coincidentally

Global Warming comparison

Anther “interesting” calc is to work out how many g/km of CO2 a cyclist produces. (it ain’t zero btw…. 😉

Well done, chaps.

Now: Strava also outputs “calories”, which I take to mean fuel burned. According to this, I burned 1473 calories on the ride. This is 6163KJ.

Strava:

Calories (or kcal) is a measure of the energy you’ve burned during your activity. Strava will calculate calorie data for your rides and runs. Calorie calculations are estimates based on power output and a coefficient for human efficiency.

Let’s work it out again with this figure:

Cycling: 6,163,000 Joules
Driving: 112,640,000 Joules

Now, cycling uses 18 times less energy than driving.

Cycling = 70,034 Joules per kilo
Driving = 100,841 Joules per kilo

Obvious flaws are: “energy consumed by driving” figure is an underestimate as it takes no account of elevation

Strava’s “calories” figure is an estimate. We have no way of knowing whether it’s a good estimate or not.

I eat more when I drive, usually have a hand free and bored.

As above you need to take into account energy used by driver whilst driving, which mitigates, to some extent, the cyclists “being alive energy usage”.

Wonder how the comparison is going to stack up substituting my EV? The electricity’s going to be produced from a number of sources including hydro, wind and nuclear possibly. Then there are other variables to add such as tyre pressure, breakfast etc. Got my lunchtime activity covered!

Energy efficiency only really matters if it contributes to a overall positive sustainability impact. A banana may be an efficient food in terms of energy conversion and g/CO2 but it’s not a very sustainable crop and has had negative social impacts.

I think the only way to scientifically prove his is to put a Mar’s bar in your car and drink a litre of petrol, then see which performs better.

Don’t know if I could drink a litre of petrol, how about frying my chips in red diesel?

Cheers, Steve

The “don’t forget to account for the [in]efficiency of the human engine” point is important, but also it’s worth remembering that most of us already eat more calories than we need. My commute is 30 miles but if I cycle home I don’t eat any more than I do when I drive. (I do when I ride in the morning, but that’s because when I do that I have breakfast at about 5am and I don’t enjoy going 7 hours without food.)

how about frying my chips in red diesel?

Come to Yorkshire, lad. It’s standard practice.