I gain about 2 MPG by going 55 mph instead of 70 mph. It saved me 1 gallon of gas last time I went from Houston to Austin, which was about $3.50 at the time.

 

Probably not correct, but irrelevant anyway... If you're going for max fuel efficiency you operate at a speed where you don't have to push air around regardless of the engine.

Here's an contrary link to the "55 doesn't make a difference" view, unscientific, but...

I got a kick out of the "number of vehicles that passed driver" statistic!

http://www.sfgate.com/cgi-bin/article....

 

i would like to know the amount of up hills and down hills they had...i mean if you are doing 55 down hill yeah your going to get great gas milage, but if you are going 70 uphill its going to hurt...i dont know the area but would be interested in that.

Because when i drive to Las Vegas from OC it is up hill most of the way and even going to same speed i get better gas milage comeing home from Vages then i do getting there....

now dont get me wrong i drive about 55MPH in my truck a 460 and no overdrive keeps me down there. but in my focus i do about 70MPH

and driving at 55 is not that big of a deal most of the time.

 

Good point, a accurate test would require the same trip at both speeds.

 

I don't know....I use to patrol a flight line for 12 hours a day driving a little ranger around, putting around at 25mph max speed did not seem to make a noticable difference in fuel consumption.

 

I keep my little ranger in 5th gear as much as possible. 55 is where it just quits lugging the engine and 60-65 puts the tach between 1500 and 2000 RPM.

There are a lot of roads around me with what are called "hollers" (hollows?) which are long rolling hills and valleys. Average speed tends to be how many people drive them - clipping down one side and keeping up enough speed to roll up over the next. I've heard more than one person nearly blow their top over "POKEY drivers" that won't get with the program...

 

well on todays cars at lower speeds, because of the Cd, they dont get cooled properly, and they may not be in the sweet spot of the power band.

I have a few old mags I save and the 1980 chevy truck ad says it got 14city (I can believe) and 28 highway - which I cannot. A big warning sign goes off: tall gearing and/or super overdrive - my 81 F100 was a prime example.

IF, and thats a big if, your driving was on a flat track in a stadium, at 55mph then maybe perhaps.

mind you, those super mileage vehicles tend to look like this sh...er I mean crap:

470 MPG-Craig Vetter Fuel Economy contests, better mileage

or this

Super mileage competition nets 7,148 MPG | Hypermiling, Fuel Economy, and EcoModding News - EcoModder.com

and check out the engine sizes. Now honestly, does that look very LIKELY, let alone practical to put the wife and 2.2 kids in and grab some groceries?

and I shudder to think of electric cars ever taking hold. All they do is move the fossil consumer up the supply line to a plant that is not only less efficient than your car, but also pollutes far more, lb for lb of fuel AND, loses a significant portion of its output in transmission. ick.

There are smarter ways to do this. #1 is park the v8 unless it is needed.

 

Thats not exactly true.

Unless you operate it in space, you always push air around. If what you say is true, idling at .1mpg is the most efficient and I bet its not.

Air testing is harder to do, so lets pick something thicker: water

pick up tests in Trailer Boats Magazine where they measure the economy in water in terms of MPG at certain rpms and the speeds given. These are SHARP numbers because unless you are at idle, you are always in an overfueled condition and your BSFC is at its best.

You will note that there is an economical sweet spot somewhere in the mid range once on plane. This point is where the water flow around the hull is the smoothest, hence the number 1 bugaboo: water resistance, is at its least and it does NOT correspond to the lowest speed number (except at steerage)

Now take this to another extreme: jet aircraft. Once airborne, flying slow is actually a fuel waster due to the induced drag of imperfect laminar flow. There was some theory of 'total area management' that came out in the 50s and dictated plane shapes such that the cruise speed could be somewhere at 70-80% power, yet gave the longest range. In an aircraft, range = economy.

Now return to cars and the extensive wind tunner testing, or "why you no longer need to use wipers in the rain". At sub-laminar flow speeds, there is a lot of induced drag as the air leaves the surfaces, and this hurts economy. At higher speeds, laminar flow starts to break up and the resulting turbulence subjects the vehicle to increased drag. (and note it is NEVER linear)

now of course, auto engineers want to avoid making an airfoil out of a car, for the chord section represented by the length of an average sedan, Vr would easily occur at 60mph!

Take a car like a station wagon, even the roundy ones like my 91 vista cruiser, up to 45-50 is it pretty slippery - which is not that particularly useful, but it does experience laminar flow across the body and down the all important back window. Speed up in a rain storm so its visible - does the rear window become hard to see out of? this is because the air flow leaves the body and there is actually a resulting low pressure area in the back, which induces drag. (the cure? have someone tailgate me. its how NASCAR and big trucking works)

something to consider....at just a tad under 500hp, the restrictor plate cars in NASCAR are able to touch on 200mph. A number that any computer will show you cannot be done. well they do. ITs essentially the same principle of the new F22 Raptor in 'super cruise' - super sonic speed without burner. The bodies are modified jusssst enuf to make the cars laminar at speeds well over 170mph. Ever wonder how they get 4.5-5mpg, yet your 250hp pickup will show that at 60mph near WOT? However they are very touchy. You have heard the term 'take the wind off'? (its what essentially killed Dale earnhardt)

mind you, when the laminar flow breaks up, it does so very abruptly and violently - yes, a 65mph drive may produce VERY similar results to a 55mph one, but 70 may send it all to heck.

The slickest shape cars btw - are not always the best 'fliers'. A body like the 'boat tail riv' (riviera) with a less abrupt front end, would be PERFECT. And size does not matter. look at a 747 (or that new super air bus thingy) or a C5...

 

I know I was highly impressed with my old 89 taurus wagon - it got into the high thirties for mileage on trips even though it had a luggage rack on the roof. The rack was aero styled just like the body, it had cruise and overdrive. In town was a whole different story. The sweet spot seemed to be 70-75.

I've always wanted to have another one, but the transaxle is a weak point. I know now how to pull them, and can likely rebuild one myself. It's quite a job though.

 

Technically it is true - obviously if you're moving and you're surrounded by air (ignoring any air movement independent of of you), you will have some energy lost to moving through that air. So your energy loss due to moving air at speed zero is zero. My posts related to this were not about the overall efficiency of the vehicle (although I noted very early on in this thread that different aerodynamics could obviously change the "sweet spots" with regard to velocity), but about drag due to air resistance in the typical vehicle - which is why I said "everything else being equal" in the previous statement.

"but everything else being equal, you certainly will get better mileage at slower speeds regardless of the vehicle in question.

The super high efficiency vehicles I'm referring to are not interesting because we might someday be driving them, they are obviously experimental only...

 

ah yes, AXOD - perhaps the only tranny to spur a lawsuit.

problem was an undersized hole in a busing that didnt let the planetary get enuf oil

 

Well that started a conversation...

Anyways, here are the little dots I mentioned earlier. I verified that the dots are in the same places on four different fords I found in the driveway...
-1994 f250 7.5L e4od
-1986 f250 6.9L c6
-1991 ranger 2.3L 5sp (don't know which)
-2004 ranger 4.0L 4sp auto (don't know which)

 

Probably manual transmission shift points. 1-2nd shift at 10mph 2nd-3rd at 23, 3-4th at 27-30, carry 4th out to around 55-60 then shift into over drive. Then again, there are more dots then available gears. Maybe the manual mentions something about em?

 

My guess would be the dots have something to do with how the faceplate is installed on the cluster or how it was painted. As for slowing down to gain mileage if I drive 55-60mph with my 5.4L I get 1-2mpg better than at 65-70mph, this is on the exact same road going the same direction with identical weather. That said the speed limit is 65 so everyone goes 65-70 and I only do under 65 on the weekend when there isn't a huge rush to get anywhere. Also there is a break point where you go too slow and lose mileage, as was eluded to in previous posts. I would guess it is somewhere under 30mph. With a 460 I can only get 5-8mpg going in the 5-20mph range where it will get 10mpg or more going down the road at 55+mph.

 

That's true, but Carter gave us Joan Claybrook and the mandated 55mph markings on the speedo. I should have made my original post clearer.