If you were to design a car for optimal gas milage, slower is the way to go. There are two components of resistance that have to be overcome by power. The rolling resitance and the areodynamic drag. Power required to overcome rolling resistance is roughly proportional the speed of the car. For example if it takes 5 HP to go 50 mph it will take about 10 HP to go 100. Power required for areodynamic drag is proportional to the qube of speed. If it takes 5 HP to go 50 mph then it would take 125 to go 100 mph. Therfore a car with the above performance would require the sum of these powers

5HP rolling power + 5HP areodynamic power = 10 HP @ 50 mph
10HP rolling power + 125HP areodynamic power = 135 HP @ 100 mph.

10 HP is approximatly the required amount of power to drive the average car @ 50 mph on a level surface in no wind at sea level with standard atmosphere. So the above example is somewhat typical.

So from the cars point of view the power required is absolute minumum the slower you go. Now the engine and transmission put into todays cars are not very efficient at low speeds. This is because the engine is way off its power curve and the transmission's torque converter is slipping. Genreally a standard engine will have its best specific fuel consumtion SFC at peak torque RPM, at wide open throttle WOT, which is typically in the 70 mph range for most cars. The best SFC will occure at lower RPM's for off WOT conditions. The transmission will not lock up its torque converter under about 30 mph. so while the car is getting less efficient rapidly with speed, the drive train is getting more efficient.

I would estimate the optimum fuel economy for most US design cars occures in the 30 to 50 mph range for cars with automatic transmissions and 20 to 50 for cars with standards transmissions.

I would also say that German cars are more likely to get better milage at slightly higher speeds due to the design focus including driving on the Auto Bahn.

Stuff like variable valve timing and 7 speed transmisions allow the drive train to perform better over a wider range of conditions and therefore will result better gas milage at lower speeds.

John Roncallo

the smart driver is the best tool that aids in fuel economy.


and i can sum up how in two words; maintain momentum.

this means timing lights, coasting to a stop, letting the car roll faster on dowhills, accelerating downhill if there is an up hill after it, letting it slow down on long up hills, getting to cruising speed as quickly as possible, staying in top gear as long as possible and so on.

conversely, there are other things you can do, like increasing your air pressure, getting a tuneup, keeping your windows and sunroof closed, using your AC minimally, removing all the unecessary detritus from your trunk, combining trips, avoiding traffic (i used to go to the gym near work, then go to work to avoid being in traffic if i left later!) and so on.

__________________
'O=00=O'
bmw 2002.
long live the legend

Go too slow and it will take you too long to get to your destination, thus requiring more fuel

__________________
1985 CA 300D Turbo , 213K mi

Very good. Most people don't realize this. However, I was taught that when the speed doubles, the force of the wind quadruples (X4, not X5).

Also, does your car require WOT at 70MPH? If so, wouldn't 70MPH be your top speed?

__________________
2012 E350
2006 Callaway SC560

A dirty airplane will require more power to fly at the same speed as a clean one. In airplanes flight speed (or Mach number) is pre set (depending on a lot of variables like weight, altitude etc). The pilot or autopilot will apply the required power to maintain it. A dirty airplane is less efficient that a dirty one and will use more fuel. I agree that the effect may be negligible for a car, since the car is aerodynamically a lot less efficient and the speed is much lower, drag increase may be unnoticeable.

On the power, John is right. As the velocity doubles, the force quadruples as brewtoo says, but the power required is eight times larger (X8)

Jl

I attended a seminar by Roy LoPresti. He was a major factor in the redesign (for speed and efficiency) of the mooney aircraft (J model) and has been involved in numerous speed mods for different.

He stated that their research had shown that the advantage of a clean airplane over a dirty one was negligible. Same with bugs on the leading edges. Yes, it surprised me, too.

In my personal experience, I have never been able to notice any difference in fuel consumption (or performance) whether the plane was dirty or clean.

YMMV, of course.

__________________
2012 E350
2006 Callaway SC560

Hmmm I wonder if he would say the same thing for little pellets of ice on the leading edges of wings..... It really sucks flying around with the stall horns screaming at you everytime you try to climb more than 50FPM...

I agree the effects are negligable on a private plane like a Mooney or even a car but I wonder how it would affect things if that Mooney could do 500KTS. Oh wait the wings would be ripped off at 500KTS. Air while it does not seem like much is quite a force. Air resistance is an actual force that vehicles deal with.

On cars its the reason why so many of them are wind tunnel tested and its why we don't drive square cars. Ok whell I do drive a square car but thats beside the point...

Friends,
Most older MB's have a downshift/throttle cable to signal the trans. when to shift. If you lenghten the effective length of that cable the trans. stays in the higher gears longer and upshifts sooner at the expense of power.
dpk

Hey Mike, are you sure about the formula? I don't think it is dimensionally correct. You need a fudge factor man!
Works for my car though ...

JL

this is one of the weirdest discussions I've seen in a long time. All this pontificating and formulizing may yield you what?, 5 miles per tank? Do you really want to change your driving habits that much and stress over an extra mpg? Not to mention that your car will be covered in spit if you're the guy driving down the freeway going 40mph holding up traffic just because you want to optimize your fuel economy.

OK, with that said, when I want to optimize my mileage, I don't look too much at the actual speed I'm travelling, I try to minimize my RPM's. For example, when starting from a stop, gather speed gradually rather than punching it. I try to keep my RPM's below 2,800 at all times and it really saves a lot of gas.

But come on, who's really going to drive a $60,000 car down the highway at 55 MPH?

Thats part of the problem I think. Some of the cars here that are being discussed are $2000 cars and fuel cost is much bigger an issue since people who drive those types of cars generally have tighter budgets.

If your really looking at fuel economy a Mercedes is probably not the car you should be driving anyway as even the diesels were not designed with economy as their primary trait. A lot of the VW diesels and Japanese cars get much better fuel economy and have a much lower operating cost.

The other thing to consider to save fuel costs is to just stay home. Americans have for years not thought about the cost of fuel because its so cheap. Now that fuel prices are high we want our cake and eat it too with the same amount of recreational driving that we had in the past for the same price. We try to maximize our fuel economy so we can still do everything that we did in the past. Perhaps instead of driving around on the weekends visiting friends and going to dinner you could have dinner at home with friends? Instead of runnining to the grocery store for fresh groceries every day you might want to buy in bulk. Everything we do in our lifestyles no matter how innocent sounding has a cost one way or another. Living in the suburbs might save you some money on a house but does cost you in transportation expenses. Most people in the burbs can't exactly walk to the corner market to buy groceries every day.

P.S. To those who might get insulted by the $2000 car comment I have to point out that there are many millonares driving around in $2000 cars. The president of Ikea and one of the richest men in the world drives around in a 1993 Volvo 240 wagon just like mine. Just because your cheap does not mean your poor! Although I don't consider being cheap a good personal quality myself.

P.P.S Something else to look at that affects your fuel prices. When you buy a product at the big box store take a moment and turn it over and look where its made. hmmm China eh? With a huge portion of our imports coming from China the chinese are becoming quite wealthy and are starting to buy cars. Those cars need fuel. So the great deal your getting on the plastic crap your about to buy is not such a great deal anymore when you can barely afford to drive anymore because of fuel costs.

personally, i like to keep my pontificating concise.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

This is true for force not power. When speed doubles force quaduples (or is actually squared 2^2 = 4),. Aerodynamic force, or more acuratly drag, is a function of speed squared. But power is drag times speed. Therefore power is a function of speed cubed.

Its somewhat scary isint it. The good news is that drag is so small at low speeds that it is off the map. But above about 40 mph its all drag and it comes on quick.

John Roncallo

I forgot the WOT portion of this question.

The data reported for engines is typically reported at WOT but does not have to be. On an engine dyno the throttle postion is set usually to WOT and the engine external load is varried. Different loads will allow the engine to spin at different RPMs for the given throttle setting. The data for the engine is than maped at each load level where RPM, Torque and fuel cunsumption are measured. From the torque and RPM measurements HP is calculated. From the calculated HP and fuel cunsumption SFC is calculated.

Most cars do not require WOT to go 70 mph, therefore the best SFC map for an installed engine would have to come from a part throttle dyno run. In this part throttle dyno run the RPM where SFC is minimum would most likely be lower than the RPM for best SFC at WOT.

John Roncallo

about 20 years ago i had a 1980 datsun 210 wagon with a 1300 engine and a five speed. i bought new tires for it and thought it would be smart to go from 155 to 165 tires. you know a little better economy from a slightly higher effective gear ratio.

well, that little sucker was already pretty optomised with the stock size tires 'cause after the change it would only do about 65 flat out in fifth. it would go considerably more in fourth and if i were to acclelerate to 75 in fourth and get behind even a 240d (which i did once) the draft would allow travel at 75 in fifth easily.

and it would deliver consistant mileage over 30.

but after a trip once when i was supposed to meet someone at a particular place at a particular time and had a headwind and barely could exceed 45, i decided that such a price was too high to pay for max economy.

so there is the rare example of a car that would be wot at seventy (or less).

i never would have dreamed you could benefit from drafting an ordinary car. but the datsun was pretty bricklike.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.