I am having a hard time to explain to a Knucklehead the difference:

Say a car is traveling 65 m/h

if you pull the foot of the gas until the car reaches 50 miles per hour

DOES IT USES THE SAME AMOUNT OF GAS :

if you accelerate from 50 m/h back to 65 m/h

And please answer if you got a solid proof. This issue is becoming very hard to explain and also convincing some people.

Thank you in advance.

 

F = m*a
W = F*d
W || W
QED....FTMFW Biatch!

But having said that, I must confess that I am uncertain as to exactly what you are trying to prove.

 

I don't exactly know what you want to know, but I'll hazzard a guess! When going a steady 65mph the car will get XX mpg. When coasting it will use no fuel, as the electronic fuel injection system shuts it off untill the car gets down to 15mph. How much it uses under hard acceleration is anybody's guess, but that is what contributes to lousy average mpg.
I hope that helps??? Earl

 

 

did you take physics? sheesh. its like THE most simple physics ever.

 

does the OP know the answer of the question he is trying to convince his 'friend' of?

 

If OP stands for "old person", Then you must be refering to me! I don't know the answer, because his question is so stupid! I gave the simplest answer, because I'm sure for him, the simpler the better!!!
Earl, the old person!

 

Defensive are we?

OP = original poser...( i.e thread starter)

 

...decelerating from 65-50mph will take literally no fuel.

accelerating will cost you gas.

Take a physics class please, people aren't too embracing of spoon-feeding around here.

 

you will use more gas constantly going from 65-50-65 compared to maintaining 65mph



depending on the roads where you are, you'd save the most gas if you can anticipate the grades.




If you want proof I have about 3 years worth of gas receipts in a folder waiting to be thrown away...

 

dont need any, i already did some test.



Got 100+ miles per tank by just driving correctly.

 

Seems if he wasn't in line when common sense was handed out either.

Riddle me this batman.......
If a tree falls in the forest and nobody hears hears it, did it actually make any noise?

 

depends on whether anyone is around to hear it.
















































and if that person is ghey

 

Those smart-assess who are cracking on the guy, telling him that he should take a physics class, obviously cannot come up with anything more constructive than insults and generalities.

Those who say that keeping a car going 65mph uses less gas than coasting to 50 and then returning to 65mph do not take into account what happens to fuel economy when you exceed 65mph.

Basically, once you exceed 60mph, your gas mileage starts to decline, so the LESS time you spend doing 65mph, the BETTER will be your gas mileage, all other things being equal -- like distance travelled, for example.

Everyone is assuming a rapid return to 65...but what if that is not the case? Suppose the time his car travels between two points is irrelevant?

Let's say that the distance is 10 miles.

The amount of throttle required to keep a car going 65mph can be exactly the same as the amount of throttle required to return to 65mph from 50mph. The distance travelled is still 10 miles, but what has changed is average velocity.

A car averaging 65mph over 10 miles will use MORE gas than a car averaging 58 mph over those same 10 miles.

A car's average speed in this scenario is determined solely by how far it can coast from 65mph to 50mph. Since we know that the amount of throttle does not have to change when either driving a constant 65mph, or accelerating back up to 65mph, what does change is the time that this particular amount of throttle is applied.

Don't believe me? Get a car...any car...with a trip computer in it that shows real-time gas mileage. Get out on a highway where there is no other traffic to slow you down and set the cruise control when you hit 65mph. Wait a mile, reset the trip computer, and then record the average mileage achieved by driving 65mph for 10 miles.

Repeat the loop, only this time at the midway point CANCEL the cruise, let the car coast down to 50mph, and then press the RESUME button. After going 10 miles, record the average gas mileage.

 

I have no problems with that. All you basically saying is that using a very small FORCE to accelerate the car (10 miles to go from 50 to 65) could indeed result in better mileage.



All it translates to is: everytime you need to accelerate, then you obviously need a force, which in the car is the torque, which comes from the engine. Like you pointed out, if you have just anough torque to overcome drag/friction, you will accelerate in a very small ratio.


also, you have to take in consideration the fact that drag is related to the square of the speed, so your point is even more accurate, because taking more time to go from 50 to 65 will imply that wind drag is minimal.


Got what you said. It is clear.


But coming back to the original question, where the he** are you trying to get? Can you better rephrase the question??

when you deaccelerate, there is minimal gas consumption, whereas when you pull it back to 65, you need a force and that requires energy (gas).


Now, the amount of force depends on how fast you want to get to 65, as very well explained by Dr RJP

 

I wouldn't be talking about good gas milage when what I see works out to 9.7MPG?????
My '84 wagon gets 18 in town, with the air on!
Earl

 

Thanks for disagreeing and agreeing with me on both extremes at the same time



ps: The last thing I would believe are the computers. Calculating mileage by the number of miles driven/amount of gas used has shown most of them to be off around 12.5%.

 

Just curious. Isn't the engine still running/idling when you let off the gas? Or are you saying that the wheels/transmission are pushing the pistons when coasting and technically the engine is off? Because if so, I did not know that.

 

Maybe, I'm confused or everyone else is confused.

The original poster from what I got was trying to compare the gas used between...
accelerating from 50 to 65 and decelerating from 65 to 50

My other assumptions are that the person keeps the car in gear/drive and doesn't use the brakes. Also, the person spends the same amount of time accelerating as decelerating.

The difference here is the engine being driven by the transmission/momentum vs. the engine driven by the gas pedal. If you push on the gas pedal you use more gas as opposed to just keeping engine rpm high. In other words your engine speed isn't the only thing that determines your gas mileage. The force of friction as well as wind resistance is causing the actual act of deceleration; where as the act of acceleration is caused by the burning of additional fuel.

 

That's correct! My '84 factory manual states the fuel is shut off during decileration untill the speed reaches about 15mph!
I'm sure all other injection systems work the same.
Earl

 

Maybe I don't know something, but I find it hard to believe. What if you decide to depress the clutch, does your car simply stall? Not likely, which means engine is still injecting fuel, just to keep it running...

Anybody can clear this out?

 

Re-read my reply.

I made the case for the deceleration time and acceleration time being radically different, with acceleration time being longer than deceleration because I suggested applying the same amount of gas pressure for accelerating as is required to maintain the car at 65 mph.

We'll assume the car is decelerating in top gear with the engine running somewhat above idle speed. In other words, unless you shut your engine off, it is still burning fuel. In fact, with just your engine at idle speeds, your car would go around 10-15MPH without any additional input from you.

To make a small correction to your statement, acceleration is the act of overcoming friction (and gravity) -- which are the main contributors to a loss of momentum.

The biggest energy requirement in this scenario is getting the car back from 60 to 65, because, as I mentioned, 60MPH is the asymptotic break point for fuel efficiency (on average).

 

it IDLES... and idling means it is running on gas..... minimal but still burning.

 

The fuel is shut off untill the vehicle speed, or the engine RPM get down to almost idle speed! An auto trans, or a manual, it makes no differance. If you'r going 35mph and de-clutch, or put the auto in neutral, when the engine nears idle speed the injection system will keep it at idle speed! Earl

 

^ So are you saying that I can turn the car off (don't think it would allow me to while moving but assuming that it did...) It's pretty much the same thing as if it's running while coasting with no throttle applied?

 

As long as the transmission remains disengaged (as if you were to put your call into Neutral or keep your foot on the clutch), then it really does not matter whether or not your engine is running as it would no longer be a drag on the drivetrain.

You would not be saving gas by turning off your engine and turning it back on, though. At least, not in the space of time it takes a car to decelerate from 65 to 50.

 

constantly speeding up and slowing down is

If you're concerned about getting somewhere and concerned about gas, go 65mph constant.

If you're concerned about gas mileage and not so much about getting somewhere, find your most efficient speed and travel that at constant.

If you're concerned about getting somewhere and don't care about gas mileage, floor it.

/thread

 

You have to re-write this or reconsider it... it does not sound correct. Going downhill, on neutral, the car will be IDLING and it is not the "same as shut down or off". That is a fact.


Now, whats is confusing is not how much gas is being burned during neutral going down the hill (that is IDLING, so by experience we all know it is consuming minimal gas) but the how much is consuming under engaged motor deacceleration.


when going downhill, engaged on whatever gear, and you take your foot off the gas, is it burning gas till it reaches idling speed or NOT?

that've crossed my mind few times, and I've talked to lots of people and everybody has a different explanation. And that is such a theoretical question that even engine designers didnt know that specific question when I asked. And the only way to really prove the case either way is thru experiments: all one would have to do is read the gases coming out of the exhaust when engine deaccelerating: if it is pure hot air, it means no combustion took place and no fuel was injected. It is has CO's, NOX's and the rest of the deal, then fuel was injected.


Now, how much difference would there be if instead of idling on geared deacceleration, we had no fuel being injected at all (till idle rpm is reached)? Close to no difference. So basically I had given up the search for the answer.


Later

 

Actually no. When I go to drift pratice I do 85-90mph regularly in my nearly all stock '90 240sx. It has no engine mods, only struts and springs. I also happen to get 30-33MPG. Thats averaged as well, that includes about 100 miles of city driving. And as the 90mph implies, I drive the **** out of this car. I keep it maintained and tuned up and its gasmilage is awsome. Oh and I have a serious air leak Im NOT going to fix. It helps milage too much

This is becuase the engine is in its peak effency range. One of my friends tried this with one of his V8 cars (dont remember which, I think a camero) but anything below 70mph and his highway milage was crap. Above that when the engine made TQ his milage improved.

My 90 max also kind of displays this but I have alot of exaust leaks and my milage is like 19mpg so it doesnt matter how I drive. But on my familys old VG-auto highway milage doing 80-85 wasnt bad even with the car full and road tripping.

It really depends on the car too. Some times this is the case if the engine doesnt make any upper rpm TQ and has the right gearing for cruising at 60/65. Also my 240 gets great milage because of the air leak. LOL

~Alex

 

"Actually no" to what? A wealth of documented research versus a few anecdotal reports?

Don't think so. Take a look at this graph:

http://metrompg.com/zoom_image.htm?H...toBild%20graph

The graph shows the fuel efficiency results of eight different cars tested on the Autobahn at speeds from 55mph to 140 and beyond. They all tell the same story -- the slower you go, the better your MPG.

OK, your car gets good mileage at 90MPH. I'd bet a dollar to doughnuts that it would get even better mileage at 60mph...maybe in the 40mpg range.

If your buddy in his Camaro gets lousy mileage at anything slower than 70, then his car has some problems.

It has been my experience that people who claim to get better MPG at fast speeds do so as an excuse to drive at fast speeds.

Sorry...but with a few exceptions, cars in their stock configurations are going to be less fuel efficient the faster they go past 60mph.

If you can find me any research to prove otherwise, I'm open to it.

 

This discussion about fuel efficiency breaks down to two things: HP Peak range and air drag.


What Alex is saying is that theoretically you operate more efficiently at peak torque/HP, which is TRUE for ANY engine of any time. That is a thermodynamics principle.


Now REALITY is different, what dr-rjp is saying is that in the real world we have air drag and friction, and unfortunately air drag resistance is related to the square of the speed, so any thermodynamic gains due to HP Peak (more RPM, closer to HP PEAK, more speed) you will be penalized due to friction.


Great chances your 240SX will get even better mileage if driven slower. Try it out.


Later

 

Hill? What hill? Who said anything about a hill?

I was also talking about the differential and whether it turns any differently if a disengaged engine is running or if it is shut off.

 

I Was the one who mentioned about a hill... just so you could visualize the car coasting for a longer period of time. You dont like the hill? fine. Ignore the hill and your statement about coasting on neutral to be equivalent to have the engine shut off is still misleading. It is not the case. When coasting you have the engine idling and that is not the same as shut down.

 

Ive tried it and the best mpg is 75+ I tried for about a month driving under 75mph when I was breaking in my clutch (it was more of a requirement anyway) and my milage was only 22-25mpg. After breakin my milage went from 23-28 on usual trips going about 80-85 and when I go to the track Im back up to 28-33.

And as an excuse for speeding, its not an excuse for speeding. I have no excuse for speeding, I just do it (albit smartly and paying attention, infact Im safter when I speed because I pay attention but when Im going 65 Im relaxed and not paying as much attention)

any way back on topic

The thing is you have to balance all the varibles.Every car will be diffrent. My car happens to like 75+. My s10 sure didnt, it was the 2.2L 5 speed. It barely liked 65mph, it had terrible gearing. You just have to balance peak HP/TQ, fuel consumtion, drag and speed to get the best result.

What will really be interesting is what rpm the new CVT cars like the maxima and I think the versa is going to have one too. I wonder what RPM nissan will pick for cruising?

~Alex

 

Anyone else find this hard to believe? A 1990 car, getting 30-33 MPG, doing 90mph non-the-less?

 

If a Vette can get 28+ mpg why cant my car get 30+?

Also you are going on the stock ECU tune. All car manufacturers use 14.7 as the leanest they go. Running a perfect tune but using a cruising ratio of 15:1 and leaner gets alot better milage then a stock tune. My ECU is stock but I have a massive air leak, thus my car runs lean, no detonation just lean.

I guess Ill start getting recipts and calculating my milage on the recipts just too prove to YOU my get gets awsome milage........... Naw dont care.

~Alex

 

Good post. I always get a kick out of people that claim better mileage at high speeds. IMO, if that's the case, then there is something wrong with your car. I have been driving for 15 years and had about 12 different vehicles in that time and have NEVER driven a vehicle that gets better mileage faster than 60mph. As a matter of fact, with my 2K2 max 6-speed even with the nice highway cruising gear of the 6-speed, I have found peak mileage is around 50-55 mph. And I am **** about calculating/tracking gas mileage on every tank under all conditions.

 

So if this true then let me ask you this - if I was travelling at 70 mph in my 2K2 6-speed, would I get better gas mileage in 5th gear or 6th gear? If what you are saying is true, then mileage should be better in 5th gear because I am closer to the horsepower "peak" and drag/friction is the same because I am still at the same speed. I can't say as I have actually tried it to confirm, but I highly doubt that a higher revving 5th gear would get better mileage than 6th in any conditions other than maybe a 6th gear lug.

 

Tecnically there is still fuel being injected into the motor since the injectors are firing. It's just at a very, very small pulsewidth. So small in fact that if you have an aftermarket exhaust the exhaust tone will be exactly the same while coasting in gear whether the ignition is turned off or on.


BTW, this has been confirmed by a few people on this site using OBD-II scanners.

 

accerating will always use more gas then deccerateing

to accerate an object you most over come it inertia by adding a force

the force in a car comes from buring gas