How This is Calculated

This calculates an approximate miles-per-gallon comparison to a gas-driven vehicle. The methodology is as follows:

The user enters the weight of the rider and bike (in lbs) and speed (in mph). Weight is converted to metric mass, and speed is converted to meters per second to calculate wattage.

Power required to drive this weight at this velocity is calculated from the following formula (see the wikipedia entry for explanation of terms):

I’ve further simplified it by assuming that groundspeed (Vg) and airspeed (Va) are the same (ie: no wind). This gives the raw power needed to drive the bike and rider at speed.

Since humans are about 24% efficient at turning fuel into mechanical energy, the real power in watts needed to drive the bike and rider at speed is Raw Power / .24, which gives real power.

Watts are then converted into kilocalories per minute, and per hour (Cal/min, Cal/hr). Dividing the burn rate (in kilocalories per hour) by the speed (in miles per hour) yields kilocalories per mil (Cal/mi).

Using 31,000 kilocalories/gallon, and dividing by kilocalories per mile, we arrive at the eagerly anticipated Miles Per Gallon!

Update: Thoughts About This Calculator

Hello, Internet. Seems like someone sparked some interest in this and it’s getting some looks. I think mostly people are having fun with it. I mean who doesn’t like flipping the bird at a Prius for being a total gas guzzler? It’s fun! Nevertheless, I have seen some criticism of the methods, implementation, and downright objections to the whole premise, so I’d like to respond to some of those here.

• This calculator is not even close to accurate. It doesn’t account for your tires, or hills, or wind, or clipless pedals, or dual-suspension, or that trench coat that’s killing your coefficient of drag, or the sensuosity of your divine lugwork. The overly simplified formula I used is described below, and I even simplified that another step. But I think I explained it pretty well below, so follow the recipe and don’t forget the requisite grains of salt.
• The constant for kCal/gal of gasoline isn’t authoritative. I got it from How Stuff Works, which was one of the top results Googling for “calories in a gallon of gasoline”. I did note to myself that other results listed the value as being between 28,000 and 34,000 kCal/gal, and given that I didn’t want to spend more than about 5 minutes looking, I went with the middle figure.
• I’m not concerned with the fuel it takes to produce and deliver food. I don’t know how much gasoline it takes to raise a cow on a CAFO, or how much petrochemical fertilizer it takes to get a bushel of corn from Iowa to Hawaii. These are big, fat problems with our methods of producing and delivering food, our agricultural policies are a fucking disaster, and these problems are (obviously) beyond the scope of this simple, fun, inaccurate exercise. So to the folks that want to assert that person X driving an SUV is more efficient that person Y riding a mountain bike, eat a brimming tureen of dick. Besides which, given the abundance of overweight and obese Americans, it’s not like not riding a bike would put a dent in the production or consumption of Big Macs.
• You’re more than welcome to do a better one yourself. Start with my code if you like, or write a new one in Lisp, or pipe your weight and speed into WOPR and see what it comes up with (but try not to start a nuclear war if you can help it). I give you full permission to rip off whatever you like from here to make a better calculator. I’ll even link to it from here.
• No wagering! For all of the above reasons. You wanna know how many calories you burned going up Mt. Diablo? You curious about how far into the Alleghenies you’ll have to ride to burn off those pancakes? Get a cyclometer with a heart rate monitor. This is just a sucky piece of Javascript on a dumb blog coded up in a hurry by someone unconcerned with its accuracy. Someone is always wrong on the Internet, and the day I wrote this, it was me.

That addresses the very small percentage of people that have “issues” with the calculator. For the overwhelming rest of you, enjoy with my compliments and have fun with it.