A New Perspective on Fuel Mileage

April 16, 2010


With gas prices the way they are, most people complain about fuel economy less than 30 MPG.  This is quite understandable considering the rediculous way in which prices fluctuate, we are again past the $3.00 mark per gallon, and we are poised for $4.00 or even $5.00 this summer.  However, most people are not aware of the staggering efficiency that only 20 miles per gallon represents.  Allow me to elaborate with some basic math:

1 gallon of fluid= 231 cubic inches or a column of liquid 231″ long and 1″ inch wide.

1 mile= 5,280 feet or 63,360 inches

63,360 divided by 231 (# of cubic inches in a gallon)= 274.29

Now we know that a 231 cubic inch column is 1/274 of a mile.

Next, we must find out how thin 231 cubic inches of fluid becomes when it is stretched out to 1 mile.  Remember that the column is 1 inch wide.

1 inch divided by 274 (our fraction)= .0036 inch

At this point, we have established that 1 gallon (231″ x 1″) stretches to 63,360″ x .0036″- roughly twice the width of a human hair!

That’s just for 1 mile!  For 20 miles:

.0036″ divided by 20= .00018″

Thus, a car that gets only 20 miles per gallon burns the equivalent of a line of fuel 1/10 the width of a human hair!

A little further math (let’s roll it into a paragraph this time):

Let’s consider a car traveling at 75mph (396,000 feet per hour) or 6,600 feet per minute.  If the engine rotates at 3,500 rpm that equals 1.89 feet per rotation.  An eight cylinder, four-cycle engine fires 2 times per rotation.  This means that for every combustion stroke the car travels .945 feet or 11.34 inches.  With this, a car traveling 75 miles an hour at 3,500 rpm burns a column of fuel 11.34″ x .00018″ at 20 miles per gallon with every combustion.  Now we figure that 20 miles is 1,267,200 inches.  If we divide 1,267,200 (total inches in 20 miles) by 11.34 (# number of inches of our .00018″ column burned per combustion) we get  111,746.  This means that the car burns 1/111,746 of a gallon of fuel with every combustion.  We can then find that a 1 inch square of fuel .002 inch deep is burned for every 11.34 inches moved at 75 mph with 20mpg fuel economy.  Therefore:

If 100 feet equals 1200 inches, we divide that by 11.34 getting 105.82 combustion cycles per 100 feet, multiply that by .002″ getting .211″, and a typical office sheet of paper is around .0035″.  Then…

A quantity of fuel equivalent to a stack of about 60 sheets of paper cut into 1 inch squares moves a 2 ton machine (with passengers) 100 feet at 20 miles per gallon-  that’s a football field with 2/3 of a cubic inch of fuel!

I know there are stories abound about 100+ mpg vehicles, but they are experimental and/or impractical vehicles that can carry only one or two persons.  One day we may get there, but we should take the time amid the screams of inefficiency and environmentalism to marvel at what we have accomplished through engineering in less than 150 years.

***(I couldn’t help but mention that a Boeing 747 fully loaded with passengers gets around 32 miles per gallon per passenger- talk about fuel savings!)