Don’t Buy Hot Gasoline

As stuff gets warmer, it expands. That’s why bridges, freeways, and sidewalks have expansion joints. The gaps provide room to expand into on hot summer days.

Gasoline does the same thing. As it gets warmer it takes up more room. But “bigger” gasoline doesnt power cars father. The amount of energy in gasoline depends on its mass, not its volume. Since we buy gasoline by the gallon, are we getting ripped off when we buy hot gas?

To answer that, we need to answer a couple of questions:

  • How much does gasoline expand?
  • Do we buy hot gasoline?

Does Gasoline Expand Much?

Gasoline’s volumetric coefficient of expansion is 950 X 10-6 C-1. To put that in context, the graph below shows the coefficient of expansion for some common materials.

Impressive (or depressing) indeed. Notice that gasoline expands 26 times more than concrete.

But what does it mean? If you have a gallon of gas at 60F and increase its temperature to 90, its volume increase by 1.58 percent, or about one-quarter cup. Suppose you buy 20 gallons of 90 gasoline on the way home from work. The next morning when the gasoline has cooled to 60F, it takes up only 19.69 gallons. Assuming the price was $4.00 per gallon (yeah, right…), you effectively paid $4.063 by buying hot gasoline. Put another way, you paid an additional $1.25 to fill your 20-gallon tank.

Do We Buy Hot Gasoline?

Does gasoline get that hot? Certainly, it does in a tanker truck rolling through the heat of summer. But what about when it’s stored in a gas station’s tank buried underground? According to multiple sources, the temperature underground is relatively constant night or day. Buying gas in the morning or evening can’t make a significant difference.

What can make a difference is not buying gas right after a tanker dumps its load of warm gas, before the gas has had a chance to cool down.

What could make a bigger difference is watching how your drive: keep your speed down, don’t accelerate too quickly, or better yet (at least for saving gas), decrease your driving.

What are you doing to save your gas money?

The Economy of Bicyling to Work

I rode my bike to work four days last week and a couple of days the week before. Am I saving money? Let’s check the receipts:

  • $29.50 for bicycling gloves
  • $23.00 for bar end grips
  • $??.?? for the new helmet I need to buy today

If I had driven to work and out for lunch as I normally do for this same time period, I would have used at least $25 worth of gas.

Fortunately, these items will last a long time. Kinda like the gas in my tank. I filled it a week ago Sunday and it’s still 3/4 full. That’s not going to last because I have to drive to the airport and back this week, but I’m going to get more than the typical two weeks from a tank of gas.

That works for me.


Which saves more gas?

  • Replacing a 10 MPG car with a 20 MPG car
  • Replacing a 25 MPG care with a 50 MPG car

The answer might surprise you.

The video poses a thought question but doesn’t answer it. The answer is so counterintuitive that I thought it worthwhile to answer here.

Suppose a car goes uphill for 100 miles at 10 MPG and then goes back downhill for 100 miles and gets 100 MPG. What is the average MPG?

The car uses 10 gallons for the first leg and then 1 gallon for the second leg so the total fuel consumption is 11 gallons. Dividing 200 miles by 11 gallons results in 18 MPG, not the 55 MPG that would seem to make sense.

Now, pose the same question using gallons per mile instead of miles per gallon.

Suppose a car goes uphill for 100 miles using fuel at a rate of 100 gallons per 1,000 miles and then goes downhill for another 100 miles using fuel at a rate of 10 gallons per 1,000 miles. What is the average consumption rate?

Since the distances are equal, the average consumption rate is the average of the two consumption rates or 55 gallons per 1,000 miles. Since the total distance traveled is one-fifth of 1,000 miles, the total fuel consumed is 55 divided by 5, or 11 gallons, same as in the first question.

Apparently, Europeans have this one figured out, stating fuel efficiency in liters per 100 kilometers.

I still want the 235 MPG — oops, make that the 1 liter per 100 km Volkswagen 1L, scheduled to be available in 2010.

CanadianDriver has more pictures of the 1L.