What is the optimal hyway speed for the best gas milage?

[catmandu57]

I've always heard that 60mph was the most efficient speed for hyway driving. I heard the reason that the 55mph was instituted back in the seventies was that nixon wanted 60 and the operative word back then was not to give the son of a bitch anything he wanted.

With the advance in engines, I'm wondering if 60mph is still the best speed for the highest mpg, if click or clack are on board maybe they can answer this.

[phantom power]

1. Seems to be 60-65 for me. I'm sure it depends on the car.

[Dr Batsen D Belfry]

4. On my recent trip to Chicago

I set the cruise control to 62 mph and improved my mileage from 18 to 26 mpg.

Follow the guidelines on DOE's website and you should see better mileage.

1) Make sure tires are inflated to proper pressure
2) Don't carry anything you don't need in the vehicle
3) Use cruise control
4) Slow down
5) Feather brakes to slow down gradually when approaching traffic signals. It is more efficient to go from 20 to 30 mph than from 0 to 30 if you can time the lights better.

DBDB

[SoCalDem]

2. 55 was established way back when because it was thought to

conserve the most gas..

[Tesha]

15. "55" is not the most conservative, but what people *MIGHT* tolerate.

They didn't tolerate it, of course, being highly-entitled 'Murkans.

Tesha

[SoCalDem]

16. Where I live , 55 would be a dream come true..

Our freeways move at ..

"move 2 feet...STOP..move 2 feet STOP...move 2 feet CRASH... SIT Sit SIT SIt..read a magazine..move 2 feet..merge 4 lanes into 2..STOP STOP read a magazine..move 2 feet ..STOP.."

[MindPilot]

19. Wouldn't it be nice if we could file a class-action suit against Cal-Trans?

How can EVERYTHING they do be completely inadequate?

I would like to know how traffic engineers get to work, because I know none of them have ever seen an automobile.

[MindPilot]

20. self-delete -- accidental double post eom

[catmandu57]

28. LOL I've done that

and while we were sitting still two chips on bikes flew berween the cars at about 100mph. I was riding shotgun and they scared the hell out of me.

[Imagevision]

3. yeah me too 55 mph -- but what a drag...!

[etherealtruth]

5. Get Better Gas Mileage

"Get Better Gas Mileage

Even if you’re not ready for a hybrid, you still can improve your fuel economy. Here are some tips from experts:
Pump it up. Keep your tires properly inflated and your wheels aligned.
Go easy on the pedals. Gentle starts and easy stops save on gas.
Travel light. Don’t store unneeded items in the trunk. Excess weight means fewer miles per gallon.
Turn it off. Don’t let your car idle for extended periods. Turn it off, then restart.
Get regular check-ups. Worn spark plugs, bad brakes and low transmission fluid worsen fuel economy.
Drive 55. The optimal highway speed for fuel efficiency is 55 miles per hour."


http://archive.parade.com/2004/0111/0111_carsandtrucks.html

[TwilightZone]

6. Depends on the car, the aerodynamics, and the gearing.

For many cars, it's around 50mph. Mileage tends to drop above that.

If your car has an automatic transmission, the best mileage is usually obtained at the speed at which the car shifts into the highest gear under light acceleration.

So say the cars guys.

[ProfessorGAC]

7. If You Have A Tachometer, It's Easy To Figure Out

It's the point at which you have minimum RPM for a given speed. On most cars it's slightly above the point after which the transmission changes gears. In the case of my car, 4th gear is engaged at about 48 MPH. It stays pretty much the same RPM for the next few miles per hour. At around 52 MPH, the tach falls about 60 or 80 RPM. At this point, the demand for torque is at a minimum, and horsepower to overcome resistance is at a minimum. So, i would get my best mileage at 52 MPH.

Remember, there is no downside to going slower, from a mileage or pollution perspective, since we measure everything on HOW FAR we go for a given amount of gas. It will take you longer, of course, if you go slower, but there is a minimum power requirement point for every gear on every car.

I know exactly where those are on both my car and my wife's. (It's a couple mph higher on my wife's car.)
The Professor

[TwilightZone]

9. If your tach reading is falling while increasing speeds in the same gear,

you have a faulty tach.

The gear ratio is a constant. Only by increasing rpm can you increase speed in the same gear.

[Tesha]

18. Not in an automatic transmission...

> The gear ratio is a constant.

Not in an automatic transmission when the torque converter isn't
"locked up" (and remember, older transmissions didn't contain
any lock-up mechanism and modern transmissions uslaly only lock
up in top gear (or the highest several gears).

The torque converter provides a variable-ratio fluid coupling between
the engine output shaft and the input shaft of the epicyclic gears of
the automatic transmission. When the torque converter isn't mechanically
loked-up, it's quite possible for the engine RPM to vary quite widely
from what you'd calculate just based on gear ratios.

It's quite possible to see the secenario where the converter was
unlocked and the engine was turning, say, 3200 RPM with some slippage
(torque multiplication) occuring in the torque converter. Then, as
you stop accelerating, the engine management system decides it's
time to lock up the converter. Suddenly, the engine speed must drop
to the actual speed forced by the current gear ratio.

I'm not at all sure I agree this is an indicator of the "most
efficient speed", though.

Tesha

[TwilightZone]

42. The operative phrase here is "in the same gear".

Torque multiplication is highest at low speeds. As speeds increase, torque multiplication is reduced, eventually approaching 1:1.

The difference in torque multiplication with similar throttle positions and in the same gear between a constant 48 and a constant 52 mph isn't enough to account for a drop in engine speed. The engine speed at a constant 52mph is not less than what it is at a constant 48mph, regardless of what the tach claims.

[ProfessorGAC]

22. NOPE!

Sorry, that's not true. The shift in fluid pressure at a constant speed when minimum torque and horsepower are required can go down at a higher speed. The residual resistance at shift point and the additional torque required when at the bottom of that power curve are higher than slightly up the curve.

It's a fact and requires no faults in the tach. I understand what you're saying, but you are, alas, incorrect.
The Professor

[TahitiNut]

12. One of the things I love about my LeBaron ...

... is the onboard computer. I can monitor my gasoline mileage in real time. I maintain tire pressure and set cruise control based on that ... and find 45 to 50 mph to be on the high side of optimal. On freeways, however, that's an invitation for road rage, so I've found an efficiency "notch" at 61 mph. (For some reason, a bit slower or a bit faster is less efficient.) Very slight road grade changes and relative wind speed changes have extraordinary impacts -- causing variances from 55 mpg to 35 mpg quite easily.

[MindPilot]

17. My old Aerostar has that too

And it is AMAZING the profound effect that things you have absolutely no control over, like wind and hills have on the the mileage.

It seems like it would be fairly simple to have the car's computer to provide some parameters for optimum mileage given prevailing conditions.

Of course that would require the brain dead idiots out there to actually know something about the machine they are operating. And we all know that ain't gonna happen.

edited to make sense

[MindPilot]

8. It has less to do with speed than acceleration

A lot of automotive technology has changed since the bad old 55mph days.

Most of the fuel use is in bringing the vehicle up to speed. Which is why speed limits to save fuel is just another example a false solution. A bunch of cars in a 0-20 stop & go situation on an inadequate freeway are using way more gas and creating far more pollution than the same bunch of cars going 70.

[Tesha]

21. In your dreams, mind pilot.

> Which is why speed limits to save fuel is just another example a false solution.

In your dreams, mind pilot.

You will *NEVER* overcome the basic physical law that wind resistance
increases as the cube of your speed. Anything else that you postulate
is no more based in science than, say Creationism.

Tesha

[MindPilot]

26. I fully understand the math

My point is that an overall reduction in speed limits is a bandaid solution that doesn't address the real problem. The fuel saved by a speed reduction is more than offset by stop and go traffic and vehicles--like trucks and SUVs--with really crappy areodynamics.

[catmandu57]

10. I have to go to Kansas City tomorrow

It's about an hour down the interstate, I'll be using my wifes car, that's why I'm asking with my old junker going 65 is pretty good it seems to like 60 or right in that neighborhood.
She has a newer car though, and it doesn't take much to find it getting away with me, I'll be talking or gawking driving with traffic and be doing 75 before I know it.
Usually with my old ride people fly past and shoot me dirty looks for driving so slow, I'll hold to 60 - 65 and see how many assholes fly on past me.

[Ioo]

11. 0 is the best speed

But if you can't, they say cruse control will make the eng run as smooth as it can at any speed. The faster you go, the more gas you use.

[MindPilot]

14. Actually 0 is the worst speed

At that point you are getting exactly zero mpg. Cruise control does help a lot, but not because it makes the engine run any smoother. It makes much finer throttle adjustments than you can. It is the speed changes that use the gas, not the speed itself.

[Tesha]

23. I'm fairly certain that was an ironic statement...

...meaning that you should leave your car standing still and use an
alternate form of transportation.

Tesha

[MindPilot]

29. Cap'n Literal reporting for duty Sir!

:hi:

[Tesha]

35. :-) (NT)

[leftofthedial]

27. not true. you are getting infinite mpg.

but alas, this is Murka, so your results might be imaginary.

[MindPilot]

31. When you figure out how to move yourself from one point to another

without using any energy, let me know.

[leftofthedial]

33. moving from one point to another wasn't part of the question

once in motion, every car is different with regard to efficient fuel consumption

[MindPilot]

36. That's a given. Now assuming we are in a gasoline powered vehicle

with the engine running, (which is what I think we are discussing) how did you get to infinite mpg?

[leftofthedial]

38. obviously (I assume), "zero mph" was not a literal answer to that question

and equally obviously, no current technology provides infinite efficiency

that would require something like a wormhole or the ability to manipulate space time to make two points coincide. .

[fishwax]

39. only infinite mpg if you're not using any gas

which may or may not be the case at 0 mph. (I mean, as long as we're all being overly literal :))

[Tesha]

13. 'bout 40 or so, maybe less.

Ultimately, all the fuel you burn is converted, through friction and
Carnot losses, into waste heat. The idea is to minimize the sum of
all the frictions and the Carnot loss.

At really low speeds (for example, 0 MPH), your engine is converting
all or most of its fuel input into waste heat. So this obviously isn't
an optimum speed.

At high speeds, wind resistance dominates, and most of the fuel energy
going into your car's engine is turned into waste heat losses in the
form of wind drag. Wind drag rises as something like the *CUBE* of
the increase in speed, so this really becomes the dominant factor at
high (say, 80 MPH) speeds.

Somewhere in the middle, there's an optimum: The engine isn't wasting too
much energy to its friction losses and waste heat and the body isn't
losing too much energy to wind drag. This obviously varies from vehicle
to vehicle and depends on the engine, the transmission gearing, the
drag coefficient of the body, and the tire rolling friction, but
efficiency usually peaks out at a surprisingly low, steady speed
such as 40 MPH.

Sorry...

Tesha

[MindPilot]

24. Sounds like you really know your stuff here

What's your background?

Aerodynamics is really the key, which is a big reason why SUVs are big gas hogs. An SUV is probably going to achieve maximum efficiency (ok minimum inefficiency) at much lower speeds.

My little sports car at 70 is probably as efficient as an SUV at 30.

[Tesha]

37. Fiziks. Lots and lots of Fiziks. :-)

And more than half a million miles behind several stick-shifts,
conducting just these sorts of experiments, some in cars with
those instantaneous and trip-average MPG meters that people
were discussing above.

Learn about Sadi Carnot and you've learned most of what you need
to understand about why internal combustion engines are limited
to a certain maximum efficeincy that no "200 MPG!" carburetor will
ever manage to meet, let alone exceed.

Tesha

[OxQQme]

34. Get yerselves a Vacuum gauge

Not a cheapo. One that has a large dial. Hook it up to the engine. Mount it in highly visible spot. Drive as if you had an egg between your foot and the throttle pedal and maintain the highest vac reading. This is the optimal efficiency regardless of road conditions.

http://www.centuryperformance.com/vacuum.asp

And if your car doesn't have cruise control, get one. The two together will maximize efficiency no matter the road conditions or the actual road speed.

[Stinky The Clown]

25. 80 mph for me

I have an elderly Buick Regal (98) with 90,000 miles. We took Sparkly Jr to college last week. A 7 hour trip all on interstates. The car was loaded to the gunwals on the way down, and just Sparkly and me on the way back. The terrain was mountain foothills (Interstate 81 through Virginia). The car has a trip computer that gives both instant and average mpg readings. I was doing generally 80 mph (which was just going with the flow) all the way, generally with the cruise control set.

At the end of the trip, the average was 31 mpg.

When we drove north to Connecticut the weekend before - a trip of generally the same distance - we averaged 26 mpg. Our speed was far lower. Probably 60 to 65 mph, but I couldn't use the cruise due to traffic.

[RPM]

30. for maximum fuel efficiency - take the train that paralells the highway

if you are lucky enough to still have rail service.

[MindPilot]

32. Ding ding ding -- we have a winnah!

Many, I would venture most, of us don't have that alternative.

[catmandu57]

41. Are you kidding? This is misery

We have Amtrac between kc and st. louie, if I wanted to take a train I'd have to jump a better than nothing coal train.

[GoneOffShore]

40. Last time I did a road trip

I was running 75-80 and getting 35-40 mpg in my 2000 Saab 93 - manual trans and I hate cruise control.