Spork you misunderstood my post regarding CVTs. They provide better fuel efficiency at all vehicle speeds. Did you read the link?

As for cars getting better gas mileage at higher speeds than lower speed, I have always heard that the engines were designed to operate the most efficiently at 55 mpg. I suppose that they could be redesigned to operate most efficiently at a lower speed but then the long distance trucking industry would complain. Part of the lower mpg at lower speeds is due to more stop and go traffic. I wonder how the City and Highway fuel rates would compare if you subtracted the idling time from the City mpg?

>>Spork you misunderstood my post regarding CVTs.

No I didn't.

>>They provide better fuel efficiency at all vehicle speeds.

Well, almost all vehicle speeds.

>> Did you read the link?

Nope. But I'm pretty fammiliar with CVT's, and I don't think we have any disagreement about them.

I asked another poster if he meant to imply that we'd get better mileage if we just kept adding higher gears and going even faster. You said yes - that's how it would work. But clearly that's wrong.

>>I have always heard that the engines were designed to operate the most efficiently at 55 mpg.

I'm going to assume you meant 55 mph since very few cars can get 55 mpg. But part of my point is that you can't simply optimize an engine to get best MPG at any old speed you choose.

>>
Part of the lower mpg at lower speeds is due to more stop and go traffic.
<<

B.S. You don't get to mix apples and hand grenades. If you want to know what's the most efficient speed for best MPG, you have to drive them the same way.

Spork, as you surmise your Integra most likely won't get better fuel economy at 65+ mph than at 45 mph.

Without dissecting several of the erroneous statements appearing in this thread, let me say that as usual, people are taking liberties with isolated parts of the overall puzzle, with the usual result: misinformation.

I'm going to make some generalizations with the hope that those same people will consider things in a slightly different light and maybe redirect themselves and their statements a bit.

Two major points to consider when talking about engines and fuel efficiency. First, load on an engine is closely related to efficiency. That is, as the load on an engine is increased, the amount of fuel needed for each hp produced decreases. This is known as the Brake Specific Fuel Consumption or BSFC. It's an easy way to compare widely different engines and conditions.

However, as has been noted but stated in such a way that the opposite impression is given, work done by the engine is closely related to fuel consumption. So the more hp you use, the higher the fuel consumption despite the increased efficiency that the fuel is used.

The second point is that the BSFC doesn't vary anywhere near as much as most people seem to think. The torque peak of the engine usually corresponds to the best BSFC of the engine but the rest of the operating range normally is no more than 10% higher than at the torque peak. There is a small benefit from running the engine at peak torque but a much greater benefit from running the engine at peak load.

This is very important.

What does this mean? It means that load is much more important than rpm.

With that as the basis for understanding how to get the best efficiency from the engine, we can go back to the total energy usage of the car and determine the best fuel consumption window for a particular car.

For typical street engines, the higher the load on the engine at a given vehicle speed (the speed being the major indicator of the energy being consumed by the car), the higher the efficiency of the engine and the lower the fuel consumption for that energy usage.

This next part may not be all that clear for most but I'll give it a shot. Since hp is directly related and calculated from torque times rpm (specifically, torque times the engine rpm divided by 5252 rpm), for an engine to produce a given hp at a high load, the overall or final drive gear ratio can be "adjusted" as needed to get the target hp.

Example: a car consumes 20 hp while at 60 mph. The engine produces 120 ft lbs of torque at full throttle. To have the engine producing 20 hp at full throttle, the rpm needs to be limited by changing the final drive ratio. A simple way of calculating the rpm is as follows: 20 hp/120 ft.lbs * 5252 rpm = 875 rpm. So the gear ratio that limits the engine to 875 rpm at 60 mph would give the highest load. That's the equivalent of a 1:1 final drive ratio or roughly what an additional overdrive ratio gear on top of what is usually found in a six speed transmission.

In the real world, this would be unreasonable. The slightest change in road elevation or wind direction and speed would require a gear change to maintain 60 mph. Reducing the speed from 60 mph would require changing down to a lower gear to maintain the proper load and hp produced required of the engine.

In the real world again, the individual engine characteristics (ability to run at low rpm without misfiring or engine damage for example) would determine the optimum rpm and load. Some variables; compression, camshaft timing, intake, exhaust and head design, combustion chamber size and configuration, and (one that gets missed by most) engine coolant path and efficiency.

Hopefully this helps clarify the engine efficiency side of this issue for some. I think most can understand how rolling resistance and aerodynamics affect the other side.

Thanks OG1. That all seems clear and consistent with what I've understood. What confuses me are the graphs I've found on the web showing cars that get their best mileage at speeds as high as 70 mph. This goes completely against what I've always thought.

At 70 mph you need approximately 4 times the force as you do at 35. This means you do 4 times the work on a given drive (and are operating at 8 times the H.P.). So it seems to me for 70 mph to give you better mpg than 35, your engine would have to be even worse than 1/4th as efficient at 35 mph.

Despite the graphs I've seen on the web, I think I'm still leaning toward believing best mpg is going to be found in the 35-45 mph range (steady speed in all cases).

One of my cars was just as fuel efficient at 85 mph as it was at 55 mph (18.7 mpg). That was because of the way the engine was built (musclecar) and the final gear ratio. The engine had moderate rpm and essentially a light load at 55 mph, and a slightly higher rpm but much higher load at 85 mph. The same engine in another car and a much lower gear ratio achieved noticably better fuel economy and was more "normal" in its fuel consumption vs speed because the engine was loaded more heavily at lower speeds.

I didn't mention it because I felt that car was a special case.

Do you have any links to those graphs? I'd like to have a look.

By the way, just saw an "almost" blow over in a GTP racecar without a collision. Cresting a small knoll with a car directly in front - no front downforce as a result.

So here's a page that compares MPG vs. MPH for several cars. Most are showing their best MPG in the 30 mph range. One is showing best MPG at about 50 mph - but it's only very slighly better than the mileage at 35 mph.

Post #16 in this link shows plots for a number of cars. Each seems to have peak MPG somewhere below 50 MPH (they didn't plot down further).

This one says peak efficiency is around 55 MPH, but doesn't specify a vehicle.

http://eartheasy.com/live_fuel_efficient_driving.htm

Unfortunately I can't find the link I saw last night that showed SUV's getting best MPG in the 65 to 75 MPH range. Still a little hard to believe

quote:

B.S. You don't get to mix apples and hand grenades. If you want to know what's the most efficient speed for best MPG, you have to drive them the same way.

I assume you meant P.S.; spell checkers cannot find all typos. I will try to better proofread my post.

I was not mixing apples and oranges; the City mpg posted on a vehicle is based on stop and go traffic. I questioned what the mpg would be "if you subtracted the idling time from the City mpg? I assumed the mpg would go up under similar driving conditions but lacked any data.

The discussion has been informative.

a cars engine mpg is really based on your driving, highway should be the best mpg, stop and go kills the mpg and this is true for all cars

>>
I was not mixing apples and oranges; the City mpg posted on a vehicle is based on stop and go traffic
<<

The question has nothing to do with highway vs. city MPG. The question is simply "what speed gives you the best MPG?"

So you don't get to start throwing stop and go, idling, etc. into the mix for the lower speeds.

>>...highway should be the best mpg, stop and go kills the mpg and this is true for all cars.

PLEASE go back and read the question.

and efi engine has the mpg posted hwy/city and thats what it is unless you mod it

sorry guys i did miss the point on this one but, the most eff at 55 mph is based on all of your owners manual, look at it sometime.

Twice I have brought up the lack of data regarding a comparable mileage figure for high and low speeds under equal driving confitions.

The link you provided does not have a post #16.

Where are you getting your peak efficiency figures?

>> the most eff at 55 mph is based on all of your owners manual, look at it sometime.

Nonsense. First of all, it's not the same for all cars. Secondly, it's not published in the owner's manual. If you're going to make things up and state them as fact, you should choose things that aren't so easy to disprove.


>>
Twice I have brought up the lack of data regarding a comparable mileage figure for high and low speeds under equal driving confitions.
<<

Fair enough. Then you decided to answer the question by changing the question?

>>The link you provided does not have a post #16.

Shoot it looks like I failed to post the link I refered to in that statement. I think maybe this is it:

http://www.gassavers.org/showthread.php?t=858

Here's some more data:

http://www.teamswift.net/viewtopic.php?t=23840

Darrel's post on May 10, 2006 3:22 am shows a graph that gives best MPG around 50 MPH. Most of the other data seems to suggest best MPG at significantly lower speeds - which still makes more sense to me.

And finally a guy that claims his Lincoln got better MPG at 73 mph than at 63 mph (which is hard for me to believe):

http://action.publicbroadcasting.net/cartalk/posts/list/1517409.page

He also claims his BMW motorcycle got better mileage at 70 mph than at 55.

It's really tempting to put a flow meter in my Integra and do my own tests. Anyone know if these are easy to come by?

I believe that an engine probably would use fuel economically between 45-55 mph. Because there is more air resistance at 65 mph and above, and because on older cars with automatics, they operate at a higher rpm at that speed.

I don't know if the next paragraph makes sense or is correct as I don't much about fuel economy stuff. But I'll try.

I believe that engine size is also a factor, besides that fact that a v8 will use more fuel than a v4, the v8 will probably be able to go down the road at a lower rpm thereby putting less stress on the engine, which should make the engine last longer, but the engine should be using less hp to maintain the speed compared to the v4. So cylinder per cylinder, the v8 should need less hp to keep the speed constant than the v4. So the v8 should be able to use less fuel per cylinder to produce the same result, however it will end up using more fuel because it has more cylinders.

But if you want fuel economy, get a diesel. They are extremely efficient because a gasoline engine has to burn fuel at the stoichiometric level, so as perfect of an air-fuel mixture as you can get. While a diesel burns fuel very lean (more air than fuel) and is very short of the stoichiometric. There is also a point (the smoke point [which is still well away from the stoichiometric]) that the diesel engine burns fuels less efficiently, thus creating black smoke, however, great power gains can be made from meeting or exceeding the smoke point.

I myself drive a diesel CAR, a 1986 Mercedes-Benz 300SDL, and I get excellent mileage. I drove about 70 miles (at freeway speeds [about 65-70 mph]) with the gage just above 1/2 and when the trip was over, it was just below 1/2.

>>
I believe that an engine probably would use fuel economically between 45-55 mph. Because there is more air resistance at 65 mph and above...
<<

But I could make that same statement about 30-40 mph - right?


>>
I drove about 70 miles (at freeway speeds [about 65-70 mph]) with the gage just above 1/2 and when the trip was over, it was just below 1/2.
<<

I was all eager to hear your numbers - but that may be the least quantitative statement I've ever seen on this forum.

The speed determines how much energy you need. In theory, the faster you drive, the more fuel you need. But the internal combustion engine has no fixed fuel consumption to power ratio. At low loads and at very high loads, the engine wastes a lot of fuel compared to it's mechanical energy output.

Since the load on the engine depends on the speed, there is a point where the engine is very efficient and the air drag not too high to cancel out that extra efficiency. It mainly depends on the engine size and gear ratio at which speed you got the most mpg.

if you got a torque converter, you got an additional "energy sink" that converts the engine power to heat. Basically, the higher the load, the more efficient it is.

You got three non-linear factors for a stick shift car:
- efficiency of the engine, kind of upside-down "V"-shaped
- air drag, a rising parable
- losses from friction in transmission and in wheels, about "/"-shaped but not really linear

If you add all curves together, you got a very complex and kind of chaotic curve which has it's minimum somewhere near the middle. The car industry tries to set that minimum onto the most common speed limit but it is impossible to perfectly match it.

I'm sorry that I wasn't more specific, my car is a six-cylinder turbo diesel, it weighs somewhere between 3,500 and 3,600 pounds. It has a four-speed automatic transmission, and is rear-wheel drive. My car is a California version, so it has 143 hp, and I'm not exactly sure about the torque, but I think that it's somewhere around 200 lb-ft. (give or take a few). It is 208.1 in. long, it has a drag coefficient of 0.36. I believe that the fuel tank capacity is 23.3 gallons (about 3.3 is a reserve). And on that 70 mile trip I got around 25-30 mpg at 65-70 mph. Also, at 70 mph, it runs at 3000 rpm. At 65 mph it's about 2,600-2,800 rpm (I can't remember which).

I guess that you could say the same about 30-40 mph, but I think that at that speed we have to consider a mechanical fan, which the modern (I'm not sure about the older ones) ones have a clutch so the engine can drive it at multiple speeds. If the engine is running hotter at 30-40 mph, the fan will spin faster and therefore take more power to run and thereby reduce fuel economy.

Does adding acetone to your fuel increase mileage? Check out http://www.realtechnews.com/posts/2598

Use the FIND button to find the correct topic to ask. But it was asked many times already. Use FIND to read the answers.

Modern cars use electrical fans controlled by a thermostat in the radiator.

Older cars used to have a direct coupled fan and the latest belt driven fans used a magnetic clutch that divides the RPM by 2 if the load on the fan becomes too great. But I can't think of any car built in the 90s that still got a belt driven fan.