RPM and MPG

[JT95]

Okay, I know the preaching has been ringing on this for years, but I have really noticed a HUGE difference in my mileage when going at higher speeds on the big road. 10mph seems to drain 5mpg difference on my 95 Legacy. It's so dang easy to speed in my wagon because the thing is so smooth and quiet at 80mph, but 20-21mpg is the best i can do in it. If I drop to 70ish and keep the RPM below3000, I swear I kick the numbers up 5mpg. Is that the same for everybody else? I knew, of course, going faster hurt mileage, but I never would have thought it made that drastic of a difference...

[Bad Brat-No Cookie]

Okay, I know the preaching has been ringing on this for years, but I have really noticed a HUGE difference in my mileage when going at higher speeds on the big road. 10mph seems to drain 5mpg difference on my 95 Legacy. It's so dang easy to speed in my wagon because the thing is so smooth and quiet at 80mph, but 20-21mpg is the best i can do in it. If I drop to 70ish and keep the RPM below3000, I swear I kick the numbers up 5mpg. Is that the same for everybody else? I knew, of course, going faster hurt mileage, but I never would have thought it made that drastic of a difference...

 

At 65 I get 31ish MPG, at 70 I get 26ish, at 75 I am down to 21ish, and over 80 it goes down exponentially, I drive 75 or so miles each way to work in my 85 brat, and have been monitoring my gas mileage religiously for the last 3 weeks with gas prices the way they are... I have put 1776 miles on the brat as of 5am this morning since 8-31-05... and averaged 26.64 MPG over that mileage...

for me 65=3100RPM, 70=3500RPM,75=3900RPM, and so on...

[Dickensheets]

Aerodynamically speaking, if you double your airspeed the drag will multiply by a factor of four. Imagine the mileage you'd get if you drove 50mph on the freeway. If you have a headwind on the freeway slow down to salvage MPG, and if you have a tailwind, well let the ponies run.

 

Ryan (flying jets when not under my roo)

[Dickensheets]

side note...

 

Those of us on the gulf coast are cringing with anticipation of Rita. Most damage will be from wind unless your talking about the low lying areas of the immediate coast.

 

A 100mph wind is 4 times as destructive as a 50mph wind (double the speed = 4x the force). You can see how even small increases in airspeed have dramatic results.

 

Ryan, drive slower

[mtsmiths]

Aerodynamically speaking, if you double your airspeed the drag will multiply by a factor of four. Imagine the mileage you'd get if you drove 50mph on the freeway. If you have a headwind on the freeway slow down to salvage MPG, and if you have a tailwind, well let the ponies run.

 

Ryan (flying jets when not under my roo)

 

He's got it, aerodynamic drag is an exponential curve.

This was the functional basis for the late, unlamented double nickle.

 

I figgered this poster was an aviator even before I saw his sig.

 

You can't break the laws of physics ... or gravity.

[Tiny Clark]

I think most EPA estimates are based on 55-60 mph.

[cookie]

my best mileage ever was going around the corners of Big Sur at 27.75 if I recall. If I could only stand to do 55 on the highway......

[1 Lucky Texan]

side note...

 

Those of us on the gulf coast are cringing with anticipation of Rita. Most damage will be from wind unless your talking about the low lying areas of the immediate coast.

 

A 100mph wind is 4 times as destructive as a 50mph wind (double the speed = 4x the force). You can see how even small increases in airspeed have dramatic results.

 

Ryan, drive slower

 

In the case a of a hurricane, I'd say the potential for damage is MORE than the square increase in force due to increased mass of flying debris!

[rideabmx]

Aerodynamic drag is not an exponential curve; it is parabolic. Others are correct, doubling the speed will increase the drag four-fold. More generally speaking, the drag increases at the square of the velocity (so a three-fold increase in speed will result in a nine-fold increase in drag).

 

If drag were exponential, a two-fold increase in speed would increase the drag 7.38906 times, but this isn't the case (luckily for us).

 

http://www.grc.nasa.gov/WWW/K-12/airplane/drageq.html

[Gnuman]

This is exactly why I put a deflector on the back of my wagon. at the same speed (80MPH) I get 7 more MPG with a clean top rack than with luggage up there. I attribute this to the deflector actually getting the air it needs t break the vaccum bubble at the back of the car. the faster you go, the bigger the bubble, and the more drag it produces. the deflector is designed to throw air down the rear windshield to keep it rain free and dirt free (mostly, anyway). that air is also pulled into the vaccum bubble, reducing it's size. As you go faster, the deflector throws more air down the car, reducing the rate that the vaccum bubble expands. Spoilers work in the same way, for the most part. The extra large ones like on the Sti are for balancing the weight distribution between the wheels, in order to give better handling at high speeds. THis is a good thing on a track, but fairly useless on public roads as the speeds you would have to be going to get any real benefit out of the thing are so far beyond the legal limit that if you get puled over at those speeds, you can kiss your license goodbye. . .

[TheBrian]

Rather than doing actual homework, I was inspired to solve for the optimum driving speed, given the price of gas and the rate at which I value my time.

 

Say, for example, that I value my time at $10/hr, and my gas at $3/hr. Assume also that I get 28mpg at 75mph, and that my fuel consumption varies with speed as F(v)=kv^2. With these numbers, I find my minimum driving cost at $.23/mi for fuel and time at 64 mph. Wow. I've been driving 80mph all the while, where gas costs a whopping 50% more per mile. And I'm only getting there 20% sooner.

 

Conclusion: I have to get in a Justy, a Civic, or a custom-chassis lawn-mower based minicar, where I can easily afford to drive 80mph.

 

P.S. at $5/gal, I'd drive a Legacy at 54mph. In theory. Fact is, I'm willing to pay $5/hr more for the entertainment value of driving at attention-holding speeds. So I'll still drive 80 when gas hits $5/gal.

[peiboy]

I have become a MPG freak, and I am trying numerous driving habit changes to see how it affects my Mileage. Here is some data for everybody to think about

 

2003 Baja Automatic

 

70 MPH cruise = 24 MPG

65 MPH cruise = 26 MPG

60 MPH cruise = 28-29 MPG

55 MPH cruise = 33.5 MPG!

 

Too bad I can't drive 55! But the speed limits here are 60, so I have backed down to that. Given the fact that I drive 60 miles a day, for let's say 250 days a year - I should save $250 for the year.

 

Now, I did an experiment with the tailgate up and down while I was typically driving 70 with NO effect. Has anybody with a Baja tried similar tailgate trials? What were your results?

 

Thanks

[1 Lucky Texan]

Rather than doing actual homework, I was inspired to solve for the optimum driving speed, given the price of gas and the rate at which I value my time.

 

Say, for example, that I value my time at $10/hr, and my gas at $3/hr. Assume also that I get 28mpg at 75mph, and that my fuel consumption varies with speed as F(v)=kv^2. With these numbers, I find my minimum driving cost at $.23/mi for fuel and time at 64 mph. Wow. I've been driving 80mph all the while, where gas costs a whopping 50% more per mile. And I'm only getting there 20% sooner.

 

Conclusion: I have to get in a Justy, a Civic, or a custom-chassis lawn-mower based minicar, where I can easily afford to drive 80mph.

 

P.S. at $5/gal, I'd drive a Legacy at 54mph. In theory. Fact is, I'm willing to pay $5/hr more for the entertainment value of driving at attention-holding speeds. So I'll still drive 80 when gas hits $5/gal.

 

I guess Bill Gates needs hyper-drive!

 

or

 

does this mean I can break the sound barrier if I quit my job? (actually, you'd hear my WIFE's sonic boom if I quit my job!)

[idosubaru]

my 97 OBS is similar to what you're saying in terms of decreased mileage at higher speeds. though i don't see a 5 mpg difference, but that's the wifes car and on highway trips i don't typically stay around 60.

 

my older XT6 models (i have 4 that i drive, all 6 cylinders) get better mileage at highway speed. i drive 120 miles round trip everyday. at 75/80 mph my XT6's get around 27 mpg (6 cylinder, AWD). if i drive 55-60 i will not see any better mileage, it stays about the same. my first XT (4 cylinder FWD) would see top mileage around 70/75 mph as well and not get significantly better at lower speeds. that thing busted out 40 mpg, which is why i just bought one and am rebuilding the motor. the XT had the lowest drag coefficient of any car at it's time of manufacture so that doesn't hurt it. i'm an aerospace engineer and i'm sitting at NASA right now, so i understand all the principles behind aerodynamics. drag is one piece of the puzzle. if drag were the only principle behind gas mileage then driving in the highest gear at 35 mph would offer the best mileage on those principles, but that is not the case.

 

on the highway my Ford F150 gets 15 mpg and deviates little even when i'm towing a car. the drag of another car must be quite large, but my mileage will be hardly different than non-towing highway miles.

 

there are mileage variances between makes of cars. the newer subaru's, including my 97 OBS experience the "better at lower highway speeds" syndrome.

[frag]

In the light of all that has been said here about the relation of drag to fuel consumption, would it not be a good idea to have an air speed gauge or meter? That would permit one to go a little faster without compromising fuel economy when a tail wind is available.

Is such a gauge readily available or is it only an airplane thing?

[cookie]

aero is not the only factor. I have had busses and trucks that had larger motors but got better mileage with superior highway gearing. An example right now is that my friend Doug's Corvette gets better mileage than my tiny six cylinder BMW M Roadster. He has a six speed and a high speed rear end and I have a five speed and a 3:23. My car has no overdrive at all and only a five speed, it turns up like a Subaru at highway speeds.

[idosubaru]

That would permit one to go a little faster without compromising fuel economy when a tail wind is available.

Is such a gauge readily available or is it only an airplane thing?

 

practicality isn't there. one factor is that planes fly at altitudes of sustained high wind speeds. that isn't the case for us on the ground. getting something like that to function efficiently and usefully would be a task.

[cookie]

on the dash that do proctically the same thing. If you can look at the dash and see your MPG it might be pretty helpful. My stripper Forester has nothing like this of course, I'm lucky I have a fuel guage.

[mtsmiths]

In the light of all that has been said here about the relation of drag to fuel consumption, would it not be a good idea to have an air speed gauge or meter? That would permit one to go a little faster without compromising fuel economy when a tail wind is available.

Is such a gauge readily available or is it only an airplane thing?

 

Well, I suppose an airspeed indicator could give you the relative wind (give you a comparative figure adjusted for head or tail winds), but in the main roads rarely run in the same direction to cause a loss or gain. Also, it wouldn't be much of a factor until the winds were steady at over ten knots.

 

That said, if you wanted to do it for curiosity's sake ... there are a number of handheld windspeed indicators available, and I guess you could mount on on the hood. Figuring out the plumbing to provide a reliable pitot-static system to mount a discrete pitot tube on the front so you could install a standard airspeed indicator in-cabin would be problematic.

 

Look in ebay motors, the aviation section. Expect to pay between fifty and a 'C' note for either a hand-held, or a non-certrified gauge removed from some panel. We useted to have an old altimeter in my Dad's Buick, but it just sat on the dash when we took X-CNTRY trips.

[malamute]

Conclusion: I have to get in a Justy, a Civic, or a custom-chassis lawn-mower based minicar, where I can easily afford to drive 80mph.

 

I have parked my Discovery and am now using the Justy for transportation. It's either I gas up at $67 a tank or $20 for the Justy. Easy math. And if I really need a 4wd vehicle with a low gear, I have one on tap.

 

Matthew

[nipper]

I have become a MPG freak, and I am trying numerous driving habit changes to see how it affects my Mileage. Here is some data for everybody to think about

 

2003 Baja Automatic

 

70 MPH cruise = 24 MPG

65 MPH cruise = 26 MPG

60 MPH cruise = 28-29 MPG

55 MPH cruise = 33.5 MPG!

 

Too bad I can't drive 55! But the speed limits here are 60, so I have backed down to that. Given the fact that I drive 60 miles a day, for let's say 250 days a year - I should save $250 for the year.

 

Now, I did an experiment with the tailgate up and down while I was typically driving 70 with NO effect. Has anybody with a Baja tried similar tailgate trials? What were your results?

 

Thanks

 

I think the Baja is too short for the tailgate to have a real efffect on it.

 

Joe

[bulwnkl]

Like Gary said, aero drag is not the only factor. In fact, I don't think it's much of a factor at all with the newer Subies. If they'd gear the stupid things higher, I don't think you'd see nearly so drastic a change in mpg for a 5-mph change in speed. New Mazdas appear to suffer the same malady. Our '00 Neon, though, doesn't vary more than 1-2 mpg whether we're going 60 or 80. >100 mph will drop it more noticeably. I also believe the AWD is a factor. {ASIDE: Why it should be I still don't know. It takes the same amount of power to turn 4 wheels at 60 mph whether you do it all through 1 wheel or through all 4, but nevertheless turning those driveshafts takes some juice.}

[nipper]

aero is not the only factor. I have had busses and trucks that had larger motors but got better mileage with superior highway gearing. An example right now is that my friend Doug's Corvette gets better mileage than my tiny six cylinder BMW M Roadster. He has a six speed and a high speed rear end and I have a five speed and a 3:23. My car has no overdrive at all and only a five speed, it turns up like a Subaru at highway speeds.

 

There also is rolling friction, There is friction between the road and the tires, big friction at the brakes, the wheel bearings etc etc etc..

 

My 98 legacy with the 2.2 and auto got 25 at over 90 and at 55 would get 32. The car did see 38, but that was when my freind who is an amputee drove the car (right leg). He got 38mpg twice. Found out it was how he accelerated to get on the highway, and he religously held the speed limit. He also let the cruise control act like a hand throttle.

There are mnany things that figure into the mialage, hence the warning "mileage may vary".

I do agree too that its too much fun to be over the speedlimit at times.

 

Joe

[mtsmiths]

I drive the cruise like a hand throttle a lot in my '95 Legacy (the column monted toggle style). But in The Pretty One's '00, the airbag mounted controls are awkward, so I don't use hers so much.

 

She rarely uses the cruise at all, and I keep telling her why I get better gas mileage ... but then I'm her husband, so what do *I* know.

[jib]

In the light of all that has been said here about the relation of drag to fuel consumption, would it not be a good idea to have an air speed gauge or meter? That would permit one to go a little faster without compromising fuel economy when a tail wind is available.

Is such a gauge readily available or is it only an airplane thing?

 

Use a manifold vacuum gage. It will tell you how hard your engine is working.

 

Jack