What determines amount and rate of gas usage?

Factual Questions
1

What determines how much gasoline is used and at what rate? Is it while the engine in running? Is it velocity? Is it distance? Is it a combination (such as: more gas will be used while the car is driving fast for a long distance compared to driving slower that same distance, and both cases more than if the car were standing)?

This is a mystery to me (being, as I am, clueless when it comes to cars).

WRS/Thû

2

Oddly enough it is governed by how far down you press the gas pedal (for any given car, which is I assume what you are asking).

There are relatively minor contributions for the engine being in tune, tires inflated correctly, extra load carried, etc, but all else being equal it’s how lead-footed you are.

3

Is that so? Wow. I would have never thought of that. I’m so in trouble.

But then what’s this about SUVs and the like “guzzling” more gas? Soes one need to be more lead-footed than in smaller cars? Or are the engines bigger?

WRS/Thû

4

I will put this in list form to keep it simple:

  1. Your engine’s horsepower - a larger engine produces more power and generally uses more gas than a smaller one.
  2. The weight of your car - heavier cars need more power to accelerate and generally use more gas.
  3. Acceleratiing - accelerating from a stop uses much more gas than cruising at a reasonable speed. The more you stop, then accelerate again, the more gas you use. That is why cars have a lower “city milage” fuel efficiency estimate and a higher one for “highway mileage”.
  4. Speed - I will try to keep this one simple. Cars have a “sweet spot” for fuel efficiency and it starts to go downhill pretty fast once you speed up after that point. At higher speeds, this is mainly due to wind resistance. A car encounters much more wind resistance at 80 mph than at 55 mph and is less fuel efficient. This relationship is not linear (the extra speed doesn’t make up for the decrease in fuel efficiency). Therefore, a 200 mile trip at 55 mph will typically use less gas than at 80 mph.
  5. Other factors - proper tire inflation helps fuel efficiency. Heavy snow or dirt on the roads will hurt it.
5

Oh, I forgot a big one.

  1. Aerodynamic properties of your vehicle. A boxy van or even an SUV is typically less aerodynamic than a regular car. The engine must work harder to push a less aerodynamic vehicle through the air so it uses more gas.
6

Fuel efficiency depends upon (1) power output over time and (2) efficiency of your engine.

Of these two factors, (2) tends to be fairly marginal in the sense that the compared to (1), the differences between one average automotive internal combustion engine and another are pretty small. However broadly speaking a moderately sized modern engine is going to be more efficient than a large older-style engine because of recent improvements in design, less internal friction, etc.

(1) is the biggie. Fuel use is depends on how much work your engine has to do. That depends on how much force it has to put out to overcome friction and effect acceleration, and how long it has to output that force for. The more weight your car has to lug around the harder your engine has to work to accelerate your car. The faster you go the greater friction losses to mechanical resistance and air resistance. And the more time you spend moving and accelerating the more time your engine has to output force.

You might say that there is an immediate quandary: should you go faster so that you spend less time with your engine running, or should you go slower, so that there is less friction?

The answer to this quandary lies in (3) internal friction and power use and (4) the respective relationships between time and work done, and friction and work done.

As to (3), a car and its engine uses a certain amount of fuel regardless of whether the car is moving or not. This is because of internal power loss through the work involved in just keeping the engine turning over and because the longer you sit in your car the more air conditioning you use etc. So on that basis, you want to get where you’re going quickly to avoid using fuel just keeping your engine running and yourself comfortable.

As to (4), work is linearly related to time. However, work is geometrically related to speed, because friction (and therefore the force required to overcome friction) increases geometrically with speed.

In plain english that means that if you go twice as fast you’ll arrive in half the time, but you’ll use more than double the fuel. So overall you’ll use more by going quicker.

These two contradictory factors mean that there is an optimum efficiency speed for any given car on any given journey, which would have to be calculated individually.

7

Now, this means that if I am running low on gas, I should not necessarily drive slower but drive with the least pushing on the accelerator pedal as possible, paying no special regard to distance, right?

Does waiting for the “gas empty” light until one fills up on gas affect gas consumption or other aspects of the car’s performance? What about filling only a little at a time, such that the gas tank is never full?

WRS/Thû

8

Without turning this into a physics problem that would challenge Noble Prize winners, I can say that id you are running really low on gas, you should try to maintain a constant speed. Slowing down then speeding up unnecessarily will use extra gas. You best gas mileage is generally going to be somewhere between 35 and 60 mph but it depends on the car.

As for more frequent fillups, that is a tough one. Having a full tank will add extra weight so that uses gas just to carry the gas around. However, making more frequent fuel stops probably means that you have to go at least a little out of your way each time you fill up. Combine that with the inconvience and the rather small benefit and I would say it is not worth it.

9

WeRSauron I wasn’t looking for it but I just came across these car other tips so I thought I would share.

10

Thanks for the info! And the link was extremely helpful. So many tips I did not know of! I must have my mechanic replace the Jupiter rings next time.

WRS/Thû

11

Does using cruise control affect anything? When on cruise, one does not press the pedal, but maybe cruise means the car does it for one.

WRS

12

You are correct, effectively, the car does it for you.

13

Yes, the cruise does open and close the throttle as necessary to maintain a steady speed. In my experience a modern cruise control will give better fuel mileage than you right foot for the simple reason that the cruise is smoother than the human runing a gas pedal. Which bring up
Smoothness. Something that does not get mentioned much in fuel mileage discussions is how smooth the driver is. Does the driver floor board the gas leaving a light? Do they treat the throttle as an on/off switch? or do they look ahead, see that the light ahead is red, take their foot off the gas and coast? Driving style can account for a 20-25% difference in fuel mileage ON THE SAME CAR.

14

Interesting. I guess I should use cruise more. I do tend to move on swiftly once the light turns green - I never understood why people were so slow once it turned green, but perhaps now I know why.

I’m surprised my car still likes me.

WRS