14.7 MPG Highway - 17.7 MPG City....How??

The thing is, he is right. Once you lose the energy going up a hill, that energy is gone forever. Once you start going down the hill you still lose energy, just not as much. Going down the hill doesn’t magically add that energy back in any way, shape, or form.

 

I just did. Again, your vehicle loses more energy going up a large hill than on flat land at same distance no matter how fast you are going. Fuel consumption withstanding.

Forget perpetual motion. No one drives down a hill without engine running. So with engine running, your still using energy: engine idle, brakes, transmission downshifting, and etc.

So if going up uses 30% efficiency ( that’s 70% lost ). Going down uses 70% efficiency ( that’s 30% lost and you’re not going to achieve 100%, sorry) that’s a 100% net loss throughout the hill process.

You don’t get the energy back. Say, it takes 1 gallon of gas to go up a hill, you used one gallon. Going down you used .5 gallons ( net save of .5 gallons ). You don’t gain the .5 back. You Still lost 1.5 gallons.

 

And yet you still seem to think that you are going to get all that energy back when you go downhill.

 

It took way more energy to get that truck up the hill than you will get coming back down.

 

Yes, you get some of it back..... Matter of fact, I was getting better mileage going up and down the mountains driving the Enchanted Circle in New Mexico than I do driving the highway here. A good bit of that is because we are driving at much lower speeds in the mountains than the highways. Wind resistance is harsh above 70mph in these trucks.

 

Yes, as I stated, you use more effiency going down, but you still lose energy. How can you get back what you’ve already lost? You can’t. You don’t get it back, you just use less.

Ok, you use one gallon of gas going up hill. Coming down, you use .5 gallons. That’s a 50% net gain over going uphill. Great. So in total, throughout the hill process, you lost 1.5 gallons. That .5 gain isn’t a gain, it’s a never was. You don’t add the .5 back.

 

I'm with you.... Mod the shit out of your truck and expect EPA rated gas mileage and were not upfront about it when you created the post.

 

Lol that's "momentum."

 

Exactly. By the chart, and in real life, every time the coaster goes up hill, it loses potential energy making its momentum slow down. Soon, without more energy applied, it won’t make it up a hill.

 

The energy that was spent actually getting the truck up the hill, will come back when you go down the hill (minus the x component)...... The problem is about 2/3s of the energy that was spent in the form of generating heat and not actually pushing that truck up the hill.

 

It’s not even partially offsetting. It’s still a “never was” type scenario. If it was offsetting or even partially offsetting, it would go like this:

Use one gallon of gas going uphill. Half gallon downhill. Half gallon saved.... use of fuel: 1 gallon. Or partially: 1.25 gallons.

The extra energy was just never used to use the last .5 gallons. You didn’t offset fully or partially.

 

F*ing Fantastic.
This made my Friday,
Thank you

 

Maybe it's because of the context of the thread.... MPG loss, which is efficiency.... I understand potential and kinetic energy at least a little bit, but that's only part of what the thread is about.

 

But the rock doesn’t have an engine that’s propelling it constantly. And while going down the hill in your truck, the engine is at least idling, but more likely the operator’s foot is on the accelerator.

 

Technically, it is the square of the velocity, as fun things happen once you leave the realm of two dimensions. But that's being overly picky for this particular situation.

A lot of good points in here. The main differentiating point, as far as I see it, is an application of a more basic type of physics understanding, vs a more refined application of physics.

Depending on the degree of simplification of the situation, everyone who posted in this thread has the right answer (small inconsistencies/verbiage used aside). But the main reason some engineers and physicists make the big bucks is because they provide you with a MORE right, right answer. And this thread is a poor place to attempt to expand upon the subtleties.

I'll end with this: If you think a 30% loss of energy (in the example of the second peak of a roller coaster to the first) is a trivial loss, try taking a 30% pay cut. I hope that brief and simple analogy sheds a bit of light on the situation.

 

The blind spot may very well be a poorly described 'system'.

Yes, throw a rock in the air and feel the 'energy' when it falls back down and impacts your hand. But are you removing your arm from the system in your example? Because the arm is most definitely a part of why the rock impacts your hand.

To me, this again is an inability to fully understand the situation, in a physics sense. Or rather, applying Physics 101 equations and limitations to a real world (graduate level) situation.

 

That seems pretty bad. Now, I'm stock for the most part, but the big dips below are when I was towing either a popup or travel trailer and even those only have gone under 14 twice....and nearly every time towing I'm heading up some steep mountain passes. Those were definitely in S4 also.

 

I just filled up my truck, squeezed in every drop I could. Started it up and the dashboard says 265 miles until empty. It would read 280-320 miles until empty when I first got the truck in September and would fill it up. Something is not right.

 

it’s not good to fill it till you can’t get another drop in it. It needs air to push as the pump sucks to run efficiently. But when I fill mine, sometimes it says 340 but it drops quickly to 320. Most of the time, it’s in the 320 range.

Did you put on bigger tires/rims? Mods to suspension and such to cause a drop like that or is it stock?

 

Totally stock.