5 Driving Tips That Will Save a Very Small Amount of Gas but That You Should Know Anyway

1. Combine errands to take a long trip rather than multiple short trips.

Combine trips not just to reduce the total miles driven, but also to do more of your driving while your engine is warm. The theoretical maximum engine efficiency increases as the temperature difference between the hot and cold reservoirs in your engine increases. The efficiency increases in practice as your engine heats up. So once you drive a few miles and heat up your car and reach the optimal temperature for fuel efficiency, you might as well do all the rest of the driving you need to do before your car cools down and you are stuck getting a lower mpg again.

Note: The efficiency does NOT increase in practice as the outside temperature gets colder, especially not in hybrid cars, since batteries do not like cold temperatures. There are many reasons winter is not good for gas mileage. Talk to me outside of class for more on this.

2. Don’t believe people who tell you that it takes a whole lot of gas to start the car.

Turn it off if you won’t be going anywhere, rather than leave it idling. Tom and Ray from Car Talk say that a maximum amount of time to leave it running, even accounting for any wear and tear associated with starting, is three minutes. (I mean, think about it. Your car couldn’t possibly burn all THAT much fuel in two seconds because there is a physical limit to what can be injected in that time with your pistons firing at a certain rate. If fuel somehow WERE sprayed in like from a hose, the heat produced would have to go somewhere, and would, like, melt your engine or cause an explosion or something.)

3. Don’t waste your money on premium gas unless your car is a special one that actually needs it.

It has recently been shown that premium gas doesn’t really have much benefit at all.

4. When picking a highway exit for a refueling stop or bathroom break, choose an exit that goes uphill rather than downhill.

     When you take an exit going uphill, gravity converts your kinetic energy (velocity) to potential energy (height), saving you wear on your brakes going up. Gravity also helps accelerate you down when you get back on the highway, saving you gas! Going down an exit doesn’t just waste your kinetic energy that you already had, but also makes you fight gravity when getting back on the highway.

Let’s quantify the advantage of taking an exit UP 8 vertical meters rather than DOWN 8 vertical meters: potential gravitational energy (PE) is mass times acceleration of gravity times height. Let’s say your car has the mass of an average mid-sized sedan, at 1600kg, plus whatever luggage you have, plus you and your passengers. On Earth, gravity’s acceleration is 9.8m/s2 and the overall height difference between 8 m up and 8 m down is 16m. PE= 1600*9.8*16= 251,000 Joules. A gallon of gas provides about 123,400,000 J of energy, of which only about 40,000,000 goes actually goes toward doing work to accelerate the car or keep it moving. 251,000/40,000,000 = 0.00627 gallons saved, which, at $3.00 per gallon, saves you just about 2 cents.

Sure, that won’t break your bank, but it is not just about you. Think of the cumulative effect of car after car going down an exit, then having to blast emissions to get back up, minute after minute, day after day, year after year. We should exclusively choose UPWARD exits and store our car’s kinetic energy in the form of gravitational potential energy just on principle. Downward exits are inherently flawed either by their design or just because of poor location choice. A resulting reduction in business for DOWN exit establishments as a result of potential energy conscious drivers’ choices is not unwarranted.

5. Put your car into neutral on long downhills. Or actually probably don’t do that.

The principle is this: Shifting to neutral on a long downhill stretch eliminates engine braking and therefore increases fuel efficiency. Detractors say that for newer cars, no fuel is used when wheel motion keeps RMP high enough, but that in neutral, RPM drops, and your engine burns fuel to idle. They also say there is a safety concern, since you will not be able to accelerate until you shift back into gear. To them I ask, “How often do you need to slam on your GAS pedal?” All I know is that in my old 1984 car, there were definitely a few long stretches of highway that I took somewhat frequently, where it was definitely worth it and fun to pop it into neutral for a couple / few miles. I am talking about where there is a sign that says something like, “4% grade next 6 miles.” Now I have a hybrid. You don’t ever want to put a hybrid in neutral since they regenerate energy on downhills. You also probably shouldn’t use neutral on hills that are so steep that you would need to use the brakes. You probably WANT engine braking for that.

6. Drive slow on the highway.

Air resistance increases with the SQUARE of velocity, so it is more efficient to not go too fast. You also want to keep your RPM from getting too high and increasing engine friction. So if you want to save gas you might want to spend a little extra time getting there and drive 60 mph rather than 75 or 80 mph.

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