Most effiecient speed

The MOST efficient speed is the speed of the traffic in the slowest lane.. and keeping it steady.

That is, unless you dont give a flyin' ____ about other people's mileage efficiency, or your or your passengers' nerves, and your time is worth nothing to you.

There is, in the grand scheme of things, no "most fuel efficient speed". Especially given that the conventional, gasoline powered, Otto-cycle engine is far from efficient.

As far as increasing speeds... the drag co-efficient increases exponentially... meaning the faster you go, the harder it is to go faster.

Operating any car will have us spending the most time in part throttle operations.... cylinder filling will be incomplete and pumping losses will be incurred.

FWIW... operating in overdrive (or the highest gear available) at roughly the rpm where peak torque is developed *should* net the best possible fuel mileage.

Lastly, an observation... your last statement comes across too much like a demand..... If you want to demand things, you'll have to pay us more....

My guess is it depends on the RPMs at which a particular cars torque and HP ratings are attained. Two of my cars have fuel computers, that indicates the fuel mileage as one drives. Both of them show the highest average mileage at around 65 MPH and 2,000 RPMs. At 55 it is around 4 MPG less, curious ay?

mike hunt

No. Which is why I say again i dont like using OD when cruising at 45 mph.

What i would like to ask Yeastwardbound is what speed he believes is proper to merge into freeway traffic.

I would bet he thinks it's a good idea to merge at 55. And make OTHER people use up THEIR fuel to adjust to him.

Peak torque on my Taurus is at 3250 rpm, in OD that would put me at about 98 mph...

Rob

I dont think he REALLY meant PEAK torque, rather the shoulder of the curve... but then, you knew that didnt you.. ;)

OK Backyard, you started this assault on Jim, so here is a bit more:

In fact, neglecting turbulence and some other 'nonlinear' effects, the drag coefficient changes very little until you get pretty close to the speed of sound. What changes is the drag force, which is proportional to velocity squared. And, as basic physics teaches, power is force times velocity, which makes it proportional to velocity cubed. Not quite exponential, but still rising pretty fast, multiplying by a factor of eight each time velocity doubles. Playing a bit with numbers for a typical passenger car (Cd=0.4, cross section of 2.5 sq meters), I get about 4hp at 40mph, 13hp at 60mph, and 43hp for those maniacs that zoom by me at 90mph on my daily commute. Add another 44hp that's needed to move 3000lbs up a 6% incline at this speed, and you can see where their money is going.

OK, now we'll probably hear from an aer>As far as increasing speeds... the drag co-efficient increases

Had a '75 Celica GT 5 speed - absolute best fuel mileage was at something like78MPH - 2400RPM in 5th where it did an honest 50MPG Canadian (40MPG American) if held steady - no accellerating to pass, or backing off (Waterloo to Peterborough Ontario at 2AM - 1980 on the way to the Tall Pines Rally.) At 55MPH 30 MPG was stretching it.

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Hey... no I didnt 'assault' Jim. Let's keep this straight.

And let's keep something in mind, here... same as I first posted: The 'most efficient' or economical speed has nothing to do with it. It's the 'proper social speed' that counts.

Bear in mind:

When speed limit was raised again after the 70's "55, stay alive" debacle, the road accident and fatality rate went down... why? Because drivers were more alert and werent falling asleep.

If you go by simply "most efficient", it's obvious that the lower the speed (given the car is geared to match) the greater the efficiency.. thus why not just set the speed limit at, say, 35 mph?

Well, that's not just boring as hell... it's bad engineering. Because one factor that SO-CALLED safety experts ignore is FLOW DYNAMICS!

Case in point: Around Columbus' beltway there's a stretch of heavily traveled freeway.... cops used to get on TV news and mouth the crap that drivers should 'slow down and leave plenty of distance from car in front'

now assume that at any given time in rush hour there are 500 cars traveling through in a 5 mile stretch between ramps A and B, at the same moment there are 50 cars, per minute, getting on at A and 50 getting off, per minute, at B. If the speed traveled is 60 mph.. the load on the freeway between the points is about 750 cars.

If the speed limit is changed to 30 MPH, you have the same 500 cars going through and the same 50 cars getting on and off, but each of those 50 getting on is staying on twice as long. Thus at any given time, you now have 1000 cars in that stretch between A and B.

The 'experts' will say that's just fine because at 30 it takes less time and distance to stop but that's where the boredom factor comes in... to say nothing of overconfidence. i dont think anyone will dispute that most accidents happen at low speeds and are caused by inattention.

Summary: Drive wide awake, pay attention and blend in with traffic... do not pick the highway as a medium to expose your anti-social behavior.

My 95 T-Bird 4.6 gets its best mpg at 55-60 mph on the interstate. 55mph, tach showing 1,500 rpm = 29 mpg

Steve

I suspect either your fuel computers are imperfect, or the driving conditions are not truly comparable. It is very unlikely that your vehicles (assuming they are stock) get better fuel economy at 65 than at 55. The power required to overcome drag at 65 mph is around 50% greater than at 55 mph. For production vehicles the bsfc is pretty flat over the normal operating range, and the difference in bfsc between 55 and 65 will be trivial (this is often not true for highly tuned engines, but I assume yours are stock).

I have a lot of experience with Ford fuel computers, and they are not particularly accurate. What technique are you using to determine the 5 mph average and the 65 mph average? When I do that, I usually press the reset button and try to drive at a steady speed for at least 10 miles. This is pretty hard to do at 55 mph, at least in my area. The roads with 55 mph speed limits are usually not conducive to maintaining a steady speed. On the other hand, maintaining a steady 65 is a snap. I have run the same roads at both a steady 55 and a steady 65 and gotten exactly the results you would expect from the fuel computer, - better mileage at 55 than at 65. A few years back, the EPA conducted similar tests, and only one vehicle got better mileage at 65 than at 60 (a 1997 Toyota Celica), and that vehicle got better mileage at both 50 and 60 than at 55. See

. Ed

You are free to believe whatever you wish.. Try driving with cruse control over the same roads day after day. ;)

mike hunt

>

The latest Consumer Reports (the car issue) had a bit about this.

For a Camry, they got these numbers:

40mpg @ 55mph 35mpg @ 65mph 30mpg @ 75mph

For a Mercury Mountaineer:

24mpg @ 55mph 21mpg @ 65mph 18mpg @ 75mph

Also remember that everything from wind, temperature, tire pressure, the road surface, hills/mountains all have an effect, too.

Those numbers seem too linear to be believable. The difference in MPG should not decrease the same number when going from 55 to 65 as they do from 65 to

75MPH. Looks to me like more bullshit form CR. Bob

Who the hell is getting 40mpg out of a Camry???

Hey BY, I agree with your post mainly. It's late so my attention span is short and i bypass longer posts. Most efficient doesn't always mean slower. In a recent RT around OHIO with my car running at it's peak (84 Mark VII, fresh tune up and fluid change, etc) I averaged 24mpg fot the 496 mile round trip at avg 68mph. The trip computer was within .5 mpg average so I trust it's instant readings within 3-4 mpg. At 35 mph, my instant mpg is 18mpg (steady throttle for a distance). Playing with the tripminder, it seems that the ol' Mark gets peak MPG at 63MPH under steady throttle. Under 45 it drops out of OD at light throttle killing the mileage. 35mph steady nets apparently

14mph on that particular car.

My '00 Duratec wagon has given me 30 mpg. at avg. 72 mph on the cruise on level road no head wind. Motors running at what rpm, I think 2.5k if memory serves me.

Gotta love that. "The numbers don't match my pre-conceived notions, therefore they are wrong."

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