Sunday, September 28, 2008

Bicycle operation cost versus car operation cost: fuel costs

It's said that bicycling is the new golf. I'm unsure whether this means that it's the new Saturday morning bonding thing-to-do or rather that it's expensive and the people who do it wear flamboyant clothes and cleated shoes. Both are applicable.

The cost of bicycling can be surprising. Of course the Saturday morning enthusiasts who listen to the salesmen in the bike shops and buy those soulless carbon-fiber-everything bikes are laying out a fat wad, but what about the commuter who wants only a good, dependable ride? The cost of operating a bicycle is an interesting topic to me for obvious reasons, and I will occasionally examine some of its many facets in future posts.

It's regularly assumed that a bicycle is cheaper to operate than a car, but is this true? and if so by how much? In this post I will explore one specific part: fuel costs. Fuel prices have increased in recent years both for cars and bicycles. We notice these increases whenever we pass those bright gas station signs boldly proclaiming their incessant faith in the one-tenth penny, and we notice these increases in the grocery store when that one pound brick of cheap cheddar costs more than five dollars.

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Case study

Subject: Craig
Description: Doesn't own car, wears a lot of Lycra, bikes everywhere around Phoenix.
Guess: Must be saving enough money not buying gas to buy an Italian racer.

I average about 126 miles of utilitarian bicycling each week. The amount sounds very specific, but it's a guess; I'm confident it's close to the actual amount, and I'm using it to simply calculations later. Most of the miles -- about 85 -- are for commuting to and from work. The remainder are for errands, social things, sports leagues, etc.

The first step is to calculate my fuel costs if I drove these miles rather than biked them.

My Mazda averaged a fuel efficiency of about 35 miles per gallon, so:

126 miles / 35 miles per gallon = 3.6 gallons
3.6 gallons * $3.50 per gallon = $12.60

Thus, my average fuel costs in a car would be about $12.60 per week.

Step two is to calculate bicycle fuel cost. How exactly does one go about calculating bicycle fuel cost? The key is determining one's energy expenditure when bicycling. This website provides a simple way of estimating one's power output to the bicycle. Only a subset of the input fields matter for calculating the power output field. These input fields are: speed, mechanical loss, air resistance coefficient, and rolling resistance. If one calculates for gradients, then the weight (both of the rider and of the bicycle) and grade fields are important, too. Gradient percentages can be tricky to calculate, and my utility bicycling is mostly flattish with the uphill generally canceling out the downhill, so in this exercise I ignore gradients. These are my inputs, as recommended by the website:

Speed: 18 mph
Mechanical loss: 3%
Air resistance coefficient: 0.0036
Rolling resistance coefficient: 0.004

These are conservative estimates. 18 Mph is a casual pace for me on the flats, and the three friction fields are on the low end according to the website.

The calculated power output with these inputs is 120W (watts), or one-sixth of a horsepower[1]. This 120W figure is my power output to the bike. Of course, some of my effort is lost as waste, just as some energy in a tank of gasoline is wasted as heat when powering a car. I estimate my efficiency rating at 20%, meaning that for every one unit of energy I output to my bicycle, I must consume an additional five units of energy. This 20% figure is a guess; the experts say that riders' efficiency rates range from the high teens to the mid-twenties, and I'm pegging myself in the middle of that range.

So how much total energy do I use for utility bicycling?

126 miles / 18 mph = 7.0 hours

(Here is the reason for the 126 miles guess: it assumes that at my average speed of 18 mph I spend a simple, round 7 hours on the bike.)

To continue:

120W output / 0.20 efficiency = 600W input
600W input * 60 seconds * 60 minutes * 7.0 hours = 15,120,000Ws (watt-seconds)
15,120,000Ws = 15,120,000J (joules)
15,120,000J * 0.0002390kcal (kilocalories) per J = 3614kcal

Thus, I must consume about an extra 3600 kilocalories each week to meet the demand of utilitarian bicycling. For simplicity I will assume that I obtain all of these extra kcal by eating quinoa, which is a cheap, complete-protein grain. One ounce of quinoa contains about 105kcal, so:

3600kcal / 105kcal per ounce of quinoa = 34 ounces of quinoa
34 ounces of quinoa / 16 ounces per pound = 2.1 pounds of quinoa

One pound of quinoa goes for about $2.50 these days, so:

2.1 pounds of quinoa * $2.50 per pound of quinoa = $5.25

Thus, my average utilitarian bicycling fuel cost is about $5.25 per week. Remember that this figure is low due to conservative estimates. Even so, my total savings on the bike are:

$12.60 car fuel cost per week - $5.25 bicycle fuel cost per week = $7.35 savings per week.

So I save about one dollar per day on fuel costs. This is if I ride at a moderate pace. And if I get my extra calories by eating only quinoa as opposed to eating something pricier, and at about 675kcal (1 meal) per dollar, quinoa is a far from average cost.

Conclusion: I'll have saved enough for that Italian racer at about the same time I'll need it just to keep up my pace.

Questions? Comments?

1 The most powerful cyclists in the world are track sprinters, some of whom can reach peak output levels of more than three horsepower!

4 comments:

amckenny said...

You have done an excellent accounting cost analysis of this. However, it would also be interesting to see what the economic cost of biking to work is.

Economic cost = Accounting Cost + Opportunity cost

Where opportunity cost is roughly defined as what opportunities are given up by choosing to bike to work.

So an opportunity cost in your example might be the cost of getting a rental car or bus pass to go somewhere that you couldn't bike to. Or perhaps, the value of your time saved in getting to your work X minutes faster by car than by bike every day. Things of that nature.

Anyway, good post, just an idea in case you felt like seeing how deep the rabbit-hole goes.

Take Care,

Aaron

Filc said...

Maybe its a good thing not everyone's jumping on their Huffy and cycling everywhere. If so, looks from your personal analysis that we'd need to grow 10-20% more food. Could we? What would that do to food prices? On the other hand, we could replace some "superslab" lanes with bike paths and use the leftover acreage for growing wheat, corn or quinoa. Hmmmmm.
Filc

cmbrandenburg said...

amckenny:

This rabbit hole is indeed deep. I have a vague intention to return to analyze further the costs of car operation versus the costs of bicycle operation, for there are many factors. As an example, a good pair of road tires for a bicycle cost about $80-100, and they give only about 3,000-5,000 good miles. Car tires cost a bit more but last far, far longer.

The opportunity costs are very difficult to analyze, primarily because they depend largely on the individual. Each person has his own quality-of-life factors to weigh.

My current conclusion on the car-versus-bicycle cost subject is your results may vary. Mainly, if you love cars but don't care much for bicycles, then a bicycle will be cheaper to operate. Inversely, if you love bicycles but don't care much for cars, then a car will be cheaper. I use myself as a case in point: I drove my car far past the point of it being an embarrassment because I viewed it as a merely a tool for transport. But I demand of any bicycle I own that it deliver a high-quality ride. I love bicycles but care not at all for cars, and I suspect that bicycling ends up costing me more.

Filc:

This is a very interesting topic. Bicycles are not the innocuous vehicles many of their adherents make them out to be. They are in fact carbon-emitting, even when excluding the process of producing a bike. Bicyclists inhale oxygen and exhale carbon dioxide. This is ecologically sustainable if you grow your own organic food, but most food in grocery stores is (1) grown with fossil-fuel-based fertilizers, thus freeing carbon hidden under ground back into the environment and/or (2) transported to the store using fossil fuels.

Diamond Girl said...

OMG. You make my brain hurt.