Efficiency speed

[BIGBLUE1212]

at what speed is a subaru 2.5L with an auto most efficient?

[uniberp]

Ooh I like this type of question. Brings out all the amateur physicists. I assume by efficient you mean best gas mileage.

 

Here's 2 answers that I hope will tweak some conversation.

 

1. The speed at which the car just shifts into top (4th) gear and maintains that speed without downshifting will give you the best gas mileage. Usually this is around 43-48 mph, depending on how hard you accelerate.

 

2. The car is 99.99 percent efficienct at 0 mph. Sitting, just evaporating a little gas and oil.

 

 

 

 

at what speed is a subaru 2.5L with an auto most efficient?

[mtsmiths]

Ooh I like this type of question. Brings out all the amateur physicists. I assume by efficient you mean best gas mileage.

 

Here's 2 answers that I hope will tweak some conversation.

 

1. The speed at which the car just shifts into top (4th) gear and maintains that speed without downshifting will give you the best gas mileage. Usually this is around 43-48 mph, depending on how hard you accelerate.

 

2. The car is 99.99 percent efficienct at 0 mph. Sitting, just evaporating a little gas and oil.

 

LOL, but if you actualluy want to GO somehwere ... our '00 does it's best at about 63 mph (picking a 'realistic' speed you can actuall live with. For real world use, the difference is minimal up to about 70, above that consumption increases rapidly.

 

But even so ... long trip mileage at 65 and below is around 30.4, at tyoical western highway speeds, 75 - 80 about 28.

 

I realize that gas is spendy but frankly I'm not gonna risk getting my @$$ smashed in (even in the slow lane) on a Montana, Idaho, Washington interstate for 2 1/2 mpg. Not to mention the sheer torture of crossing the prarie at less than overall traffic speed.

[idosubaru]

mileage begins to drop around 50/55 mph. threads like this are common, do a search and read oppinions all day long.

[WoodsWagon]

I smoked from the mississippi to the middle of wyoming at 80 in my lifted wagon and got 24-27 the whole way. My car is about as aero dynamic as a cinder block, so I'd hope you would get better.

[Dickensheets]

Watch for cues as to wind direction while you drive. If you can find a healthy tailwind then go faster w/o fuel penalty. If there is a stiff headwind, slow down.

 

rd

[nipper]

at what speed is a subaru 2.5L with an auto most efficient?

 

Using my scangauge I have noticed my 1997 OBW gets its best gas mileage at 32 mph, almost 35 mpg, As soon as you cross that line it drops fast.

 

nipper

[Commuter]

*Original post*

Air drag losses are proportional to the square of velocity.

Drive train losses are proportional to the cube of velocity.

*EDIT* Sorry, I got them reversed.

Air drag losses are proportional to the cube of velocity.

Drive train losses are proportional to the square of velocity.

*EDIT

The faster you go, the greater the losses, the more gas you consume.

At highway speeds, this becomes quite significant. Eg, the difference between 65mph and 80mph is considerable in terms of gas mileage.

 

Commuter

[uniberp]

Can you expand on that 2nd equation a bit?

 

I don't think any given car has 100x the drive train drag at 21mph that it does at 4.7mph.

 

Seems to me 99% of the difference between 65 and 80 is eaten up by air drag, plus the designed operating speed range of the car.

 

Could it be that drivetrain losses are directly proportional to 1x drivetrain speed?

 

 

Air drag losses are proportional to the square of velocity.

Drive train losses are proportional to the cube of velocity.

The faster you go, the greater the losses, the more gas you consume.

At highway speeds, this becomes quite significant. Eg, the difference between 65mph and 80mph is considerable in terms of gas mileage.

 

Commuter

[nipper]

Can you expand on that 2nd equation a bit?

 

I don't think any given car has 100x the drive train drag at 21mph that it does at 4.7mph.

 

Seems to me 99% of the difference between 65 and 80 is eaten up by air drag, plus the designed operating speed range of the car.

 

Could it be that drivetrain losses are directly proportional to 1x drivetrain speed?

 

Yes i was wondering about the second one too. It takes more energy to overcome inertia starting from rest then something already in motion.

 

The biggest fuel consumers are, starting from a dead stop, and wind drag.

 

http://auto.howstuffworks.com/question477.htm

 

The car does have 100 times more drag, but that drag is compared to 0 when standing still. So the multiple may sound impressive, at low speed it really is not.

 

 

nipper

[Commuter]

Can you expand on that 2nd equation a bit?

 

I don't think any given car has 100x the drive train drag at 21mph that it does at 4.7mph.

 

Seems to me 99% of the difference between 65 and 80 is eaten up by air drag, plus the designed operating speed range of the car.

 

Could it be that drivetrain losses are directly proportional to 1x drivetrain speed?

*EDIT* See my prior post. I had the relative influences reversed between drive train losses and air drag losses. I've revised my post below (from a few hours ago) to correct this. Thanks nipper for the link. It put me straight.

 

Bear in mind...

1) Speed refers to the speed the drive train is running at, not the car (to my knowledge). Gearing comes into it. And different parts of the drive train are running at different speeds, so what the total is, I don't know. The point is the relationship. Run the engine at twice the speed and its losses are 2^2 or 4 times as much.

2) Drive train losses are relatively small compared to other losses. You have to keep the relative quantities of the various losses in mind. Just as an example, say that drive train losses are 10% of total losses at 60 mph. If you speed up to 75 mph, you have increased your speed 25%, or 1.25 times. 1.25^2 = 1.56, or just over 50% more. Therefore, 10% of your losses has now become 15.6%. If all other losses were unaffected, then you would have gone from 100% to 105.6% on the losses.

 

Now to go thru the same example but with air drag and its cube relationship. Just as an example, say that air drag losses are 10% of total losses at 60 mph. If you speed up to 75 mph, you have increased your speed 25%, or 1.25 times. 1.25^3 = 1.95 (or almost double). Therefore, 10% of your losses has now become 19.5%. If all other losses were unaffected, then you would have gone from 100% to 109.5% on the losses.

 

http://www.fueleconomy.gov/feg/driveHabits.shtml This link shows a typical fuel economy (gas mileage) curve. Once you hit highway speeds, it starts to drop off considerably.

 

Commuter

[frag]

 

2. The car is 99.99 percent efficienct at 0 mph. Sitting, just evaporating a little gas and oil.

 

 

Is'nt that 0 percent efficiency, on the contrary?

[idosubaru]

my 97 impreza OBS sucks worse at higher speeds than my XT6 (which gets better highway mileage and has two more cylinders!!!! - both automatic awd). in other words my XT6 won't loose nearly as many mpg at say 70 mph highway speeds as my OBS does. i've only owned one OBS so i don't know if that's model difference or just mine.

[avk]

My van shows gas mileage on the overhead display and the best occurs around 45-50 mph. And that with a tall gear ratio, 0.69 for overdrive x 3.609 for final drive. The mileage goes into single digits when accelerating from a stop. The bottom line is that the optimal speed is not practical for highway cruising and in city driving, it doesn't matter. Diesels are a different story.

[uniberp]

Is'nt that 0 percent efficiency, on the contrary?

 

Well, it does provide shelter from wind and rain. That's worth something.

[frag]

Well, it does provide shelter from wind and rain. That's worth something.

You have a point there. And maybe using the back seat... would push that efficiency way up there getting real value from that stupid evaporating fuel.:drunk:

[robm]

About 20 years go, Car and Driver's road tests used to contain figures that showed the horsepower required to drive the vehicle at 50 mph. They were even able to break that down into how much HP was used to overcome rolling resistance, and how much to overcome air resistance.

 

The typical figures for a small econobox (Corolla, Datsun whatever10, Subaru of the day) was 15 HP at 50 MPH, 7.5 HP for rolling resistance and 7.5 HP for air resistance.

 

These numbers would give good starting points for the physicists in the crowd to do their calculations, then they could go out and drop the cannonballs, and find out how closely the calculated results match reality.

 

I suspect these numbers would apply better to the EA82 body style, as they are closer to the type of car that was being tested than a current EJ in shape, tire size, and curb weight. But it is still a reasonable place to start.

[skibumm100]

I would expect that my Subaru responds the same as my Saab 9000T's. The best economy is at a speed to keep the car in OD and still maintain speed with the absolute minimum throttle position required. On the Saab's that's about 38 to 45 MPH. Higher speeds require more throttle and therefore more gas. Keep in mind that how well a car coasts has a lot of impact on overall MPG. In my opinion, my Subaru doesn't coast half as well as my 97 Saab 9000T. My 97 9000T doesn't coast as well as my 1990 9000T....don't know why either. The Subaru noticably deccelerates when you take your foot off the gas. This hurts overall MPG. I believe it is one of the reasons why the MPG stinks on my 98 Leggy GT. The other is that it is AWD. I have found that the way you drive has an awful lot of effect on how good your MPG is. My wife consistently gets 3 MPG less than what I do. I consciously use the minimum pressure on the gas pedal to keep the car moving at a consistent speed. I was also disappointed in the MPG difference between my 97 9000T (2.3L) at about 25-26 MPG and my 90 9000T (2.0L) at 30+ MPG over the same route, day in and day out. Same basic car and engine, 5-6 MPG less. That decrease in MPG corresponds to the difference in displacement. This makes sense because more displacement requires more fuel to reach the same AFR. The O2 sensor tells it hom much gas to put in.... more displacement....more fuel. YMMV.

[robm]

I bet the Subaru doesn't coast as well because of the AWD. More rolling resistance. Aerodynamics might come in to the picture, as well, although at less than 50 mph, this has less effect.

 

Re: the difference between the two Saabs: Is the tire size the same? That might make a difference, if the new car has wider tires. Also, is the body style exactly the same, or could there be some aerodynamic differences, as well?

 

The engine size/fuel economy relationship is not quite a simple as you make out. An engine that is under low load does not fill its cylinders completely. It only sucks in as much air as it needs to produce the power required. If the engine is very similar, same # and size bearings, pistons, cam drive system, etc., the extra drag from larger diameter rings that cover a slightly longer stroke shouldn't be that much greater, should it?

 

My gut instinct says there is something else going on here. But my guts have been wrong before!

[skibumm100]

Tire size is the same. Properly inflated. Newer car should be slightly more aerodynamic by its looks, which don't count for much. The speeds are fairly low , < 50 MPH. I'm not sure of the difference in engine spec. Probably a bigger bore. I agree that the engine doesn't suck as much air at lower loads but under part throttle, the engine management system still has a target AFR that it's shooting for. It will tweak the fuel based on O2 readings from the oxygen sensor. Under coast, throttle closed operation, I don't believe it will try to maintain AFR. Under part throttle, it will.

[Distance Commuter]

... Now to go thru the same example but with air drag and its cube relationship. Just as an example, say that air drag losses are 10% of total losses at 60 mph. If you speed up to 75 mph, you have increased your speed 25%, or 1.25 times. 1.25^3 = 1.95 (or almost double)...

 

Yes, I'm with Commuter on this one.

Here's another view of the same thing:

 

When you look at examples of change in drag for some different changes in speed you see how dramatically it increases. The "cube" factor for air drag and the percentages of increase hold true regardless of your speed.

 

So - -

Increasing speed 20-40 doubles your speed (2X).

~~ That's an 8x increase in air drag (2x2x2).

Increasing speed from 40-60 is a 50% increase in speed (1.5x).

~~ That's a 3.375X increase in drag; nearly 4X from the 40 mph drag!

Increasing speed from 60-80 is a 33.3% increase in speed (1.33x).

~~ That's a 2.37X increase in drag.

 

Increase from 20 to 80 is 4x the speed.

4*4*4 = a 64X incrase in air drag - yikes. Unfortunately it's true.

Put your open hand out the window at highway speeds and you'll feel it. That's what your engine is pushing against, except the car's frontal surface is more like 5x6 feet or something like that.

 

So I'm also with uniberp, who posted the first answer in this thread - the lowest speed where you can be in the top gear without engine lugging etc. will be the most efficient speed. It's all downhill from there.

 

I usually drive 70 anyway. I pay for it at the pump.

[uniberp]

About 20 years go, Car and Driver's road tests used to contain figures that showed the horsepower required to drive the vehicle at 50 mph. They were even able to break that down into how much HP was used to overcome rolling resistance, and how much to overcome air resistance.

 

The typical figures for a small econobox (Corolla, Datsun whatever10, Subaru of the day) was 15 HP at 50 MPH, 7.5 HP for rolling resistance and 7.5 HP for air resistance.

 

These numbers would give good starting points for the physicists in the crowd to do their calculations, then they could go out and drop the cannonballs, and find out how closely the calculated results match reality.

 

I suspect these numbers would apply better to the EA82 body style, as they are closer to the type of car that was being tested than a current EJ in shape, tire size, and curb weight. But it is still a reasonable place to start.

 

I like these numbers. Real world constants. I wonder how the evolution of the automobile was directed by a balance of these 2 factors.

 

Cars with excess drivetrain resistance relative to windspeed rattled and twisted themselves into pieces.

 

Cars with bad wind resistance made the drivetrain work harder, again wrecking things.

 

Apparently the optimum balance point is somewhere the Subaru side of average cars.

 

That and they keep you awake at the wheel with the excess noise, avoiding wrecks.

[robm]

I don't quite follow your thinking regarding the balance between rolling resistance and air drag. Nor would I describe the figures as "real world constants." They WERE real world RESULTS.

 

One of the reason for the shift to FWD was a reduction in rolling resistance, so modern vehicles should be less than this. Subarus with AWD go against the trend.

 

Modern cars are supposed to be lower drag. Certainly, the small cars I am thnking of were called econoboxes for a reason - they had the aerodynamic sophistication of a brick. However, I have noticed that new cars seem to have more frontal area, so I am not entirely sure the total aerodynamic package is that much better than it used to be. We gave up strong roof gutters and good ground clearance in the name of fuel efficiency, but may have lost that efficiency to increased head room.

 

So these numbers are 2 decades or more out of date. But they still give a good feel for how much power is required to drive a vehicle, and how much goes to each component of total drag.

[Slegacy96]

My best is 32 mpg, crusing at 75-78 mph. All highway. Im so proud....I also have a 5 speed manual. 29 mpg was the best I got when I had te 4EAT. Which is also very good.

[Distance Commuter]

... One of the reason for the shift to FWD was a reduction in rolling resistance, so modern vehicles should be less than this....

 

Modern cars are supposed to be lower drag. Certainly, the small cars I am thnking of were called econoboxes for a reason - they had the aerodynamic sophistication of a brick. However, I have noticed that new cars seem to have more frontal area....

 

I'll just comment on a few bits from the quoted post. For better or worse, I CAN remember driving two mid- '70's econoboxes (4-spd standards) , among other cars. '76 Corolla, '75 Datsun B210.

 

I think FWD lets you cram the whole drivetrain into less space, leaving more room for passengers in a car with a smaller overall size. Smaller size = less steel etc. so you save weight, and still have a roomier passenger compartment.

 

Lots of modern cars have very low aerodynamic drag. Often you can see the spec for that on cars.com (drag coefficient). But there's a style-demand thing going on; if you want the car to look macho like most SUVs do then the designers go for a taller boxier style, big vertical grill/headlight area, and a more vertical windshield instead of a shallow slope, and the drag coefficient goes up (more resistance). For a fairly aerodynamic design, see the 2nd version Prius. A very shallow windshield slope, not very tall, and no big frontal area pushing agains the air.

 

I don't recall seeing anything saying that FWD reduces rolling resistance. Maybe in that pulling creates less resistance than pushing the front tires into the roadway? But I think that in pulling with fwd, you're trying to lift up the front so you're constantly exerting that upward pull energy. Maybe it's an even swap. In any case I suspect that rolling resistance comes in way below air drag and lots of other mechanical issues, in overall mechanical energy lost or needed.

 

Some of those old econoboxes really were boxes. Chrysler's K-cars and the VW Rabbit and the Chevette come to mind. But some were pretty sleek: Mid '70's Corolla and the ubiquitous Datsun (Nisan) B210. Especially the hatchback versions; those were pretty much a fastback like the old muscle cars - the roof curved and sloped right down to the trunk edge; the glass was in line with that. As I recall, those little things got about 30 mpg.