Ecenomical Cruising Speed of a car

Wetbench4

CiniO wrote: »

I think you didn't give a think about what you just said.

I did, trust me. :rolleyes:

Zubeneschamali

Wetbench4 wrote: »

I did, trust me. :rolleyes:

Well, the definition of better miles per gallon means you go more miles on a gallon. Speed, gearing etc. can't change the fact that more miles per gallon means less gallons per mile.

Wetbench4

Zubeneschamali wrote: »

Speed, gearing etc. can't change the fact that more miles per gallon means less gallons per mile.

Isn't this why we have a gearbox? Like the before posters said, there is a rpm sweet spot (in top gear usually) in which you are covering the most miles per gallon. Anything above or below that is not as energy efficient as the sweet spot.

edit: also this would be different on every car and even different journeys in the same car due to the huge amount of variables.

CiniO

Wetbench4 wrote: »

I think the confusion is the fact that just because your're using more fuel doesn't mean its less economical. Obviously a car will use more fuel at 120kph compared to 100kph, but that same car might use less fuel to cover 1 mile/km at 120kph than at 100kph because of gearing and torque etc.

I did, trust me.

It's impossible that car will use more fuel at 120km/h than 100km/h and at the same time will use less fuel at 120km/h to cover 1mile than at 100km/h.

Because that's pretty much what you just said.

In that case, it will use more fuel at 120km/h than at 100km/h per what?
Per minute?
You don't calculate usage per minute.
All units we use for measuring fuel consumption refers to usage per distance. (MPG, l/100km, etc).

Wetbench4

CiniO wrote: »

It's impossible that car will use more fuel at 120km/h than 100km/h and at the same time will use less fuel at 120km/h to cover 1mile than at 100km/h.

Because that's pretty much what you just said.

In that case, it will use more fuel at 120km/h than at 100km/h per what?
Per minute?
You don't calculate usage per minute.
All units we use for measuring fuel consumption refers to usage per distance. (MPG, l/100km, etc).

Its using fuel faster but is covering the ground faster and more efficiently then it would if it were going 100.

CiniO

Wetbench4 wrote: »

Isn't this why we have a gearbox? Like the before posters said, there is a rpm sweet spot (in top gear usually) in which you are covering the most miles per gallon. Anything above or below that is not as energy efficient as the sweet spot.

edit: also this would be different on every car and even different journeys in the same car due to the huge amount of variables.

There obviously is a sweet spot for most efficient RPM, but it's only small bit of the whole thing.

Take notice, that if you are parked in neutral, to keep car at 2500rpm you need to open throttle only a tiny bit (press gas pedal only small bit).
But if you want to keep that 2500rpm in 6th gear at high spead on incline you might have your gas pedal pressed nearly to the floor.

Surely consumption will differ greatly between those 2 cases.

Also take notice, that air resistance increases squarely to speed, so by accelerating from 100 to 120 (increasing speed only by 20%) air resistance will rise by 44%. That really makes a difference.

CiniO

Wetbench4 wrote: »

Its using fuel faster but is covering the ground faster and more efficiently then it would if it were going 100.

So you are saying, that at 120km/h fuel consumption per unit of time is bigger than at 100km/h, which is actually correct, but you indicate then, that at 120km/h you might do more distance with the same amount of fuel used, which means in other words that your MPG is lower.
No - trust me - I hardly believe there exist a car which is more economical to run at 120km/h than at 100km/h. And even if there is one, no one from boards.ie has one

Wetbench4

CiniO wrote: »

So you are saying, that at 120km/h fuel consumption per unit of time is bigger than at 100km/h, which is actually correct, but you indicate then, that at 120km/h you might do more distance with the same amount of fuel used, which means in other words that your MPG is lower.
No - trust me - I hardly believe there exist a car which is more economical to run at 120km/h than at 100km/h. And even if there is one, no one from boards.ie has one

Its down to where the torque and horsepower, of the engine peak. This is the most efficient rpm that the engine runs at. My original point is that because of differences in gearing etc, this is different in each model. Anything above or below this sweet spot is less efficient. Sorry, i must be crap at explaining stuff but this makes perfect sense to me. I will try and find something to back me up.

Wetbench4

Taken from wikipedia so its still not a great backmeup link but still:

Speed and fuel economy studies


1997 fuel economy statistics for various US models
Fuel economy at steady speeds with selected vehicles was studied in 2010. The most recent study[17] indicates greater fuel efficiency at higher speeds than earlier studies; for example, some vehicles achieve better mileage at 65 mph (105 km/h) rather than at 45 mph (72 km/h),[17] although not their best economy, such as the 1994 Oldsmobile Cutlass, which has its best economy at 55 miles per hour (89 km/h) (29.1 mpg-US (8.08 l/100 km)), and gets 2 mpg better economy at 65 mph (105 km/h) than at 45 mph (72 km/h) (25 mpg-US (9.4 l/100 km) vs 23 mpg-US (10 l/100 km)). The proportion of driving on high speed roadways varies from 4% in Ireland to 41% in Netherlands.
When the National Maximum Speed Law's 55 mph (89 km/h) speed limit was mandated, there were complaints that fuel economy could decrease instead of increase. The 1997 Toyota Celica got 1 mpg better fuel-efficiency at 65 mph (105 km/h) than it did at 45 mph (72 km/h) (43.5 mpg-US (5.41 l/100 km) vs 42.5 mpg-US (5.53 l/100 km)), although almost 5 mpg better at 60 mph (97 km/h) than at 65 mph (105 km/h) (48.4 mpg-US (4.86 l/100 km) vs 43.5 mpg-US (5.41 l/100 km)), and its best economy (52.6 mpg-US (4.47 l/100 km)) at only 25 mph (40 km/h). Other vehicles tested had from 1.4 to 20.2% better fuel-efficiency at 55 mph (89 km/h) vs. 65 mph (105 km/h). Their best economy was reached at speeds of 25 to 55 mph (40 to 89 km/h) (see graph).[17]
Officials hoped that the 55 mph limit, combined with a ban on ornamental lighting, no gasoline sales on Sunday, and a 15% cut in gasoline production, would reduce total gas consumption by 200,000 barrels a day, representing a 2.2% drop from annualized 1973 gasoline consumption levels.[18][a] This was partly based this on a belief that cars achieve maximum efficiency between 40 and 50 mph (64 and 80 km/h) and that trucks and buses were most efficient at 55 mph (89 km/h).[20]
However, the United States Department of Transportation's Office of Driver Research found total fuel savings of the 55 mph limit to be 1%, and independent studies found a 0.5% savings.[21]

One of example given above is the 97 celica which has two efficiency peaks on the graph and is more efficient at 105kph than at 72kph.


Here is the full wiki page: http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles

unkel

Wetbench4 wrote: »

Its down to where the torque and horsepower, of the engine peak. This is the most efficient rpm that the engine runs at.

That engine and where it is most efficient is only half of the equation. The engine is in a car which suffers air resistance when on the move. That's the other half of the equation.

If air resistance was the same at 120km/h as at 80km/h then a typical car might well be more efficient at 120km/h

But it isn't. Air resistance at 120km/h is more than twice as high compared to 80km/h

Wetbench4

unkel wrote: »

That engine and where it is most efficient is only half of the equation. The engine is in a car which suffers air resistance when on the move. That's the other half of the equation.

If air resistance was the same at 120km/h as at 80km/h then a typical car might well be more efficient at 120km/h

But it isn't. Air resistance at 120km/h is more than twice as high compared to 80km/h

Yes i agree my 100/120kph examples were bad because of drag but at lower speeds 80/100kph it makes sense.

CiniO

Wetbench4 wrote: »

Taken from wikipedia so its still not a great backmeup link but still:





One of example given above is the 97 celica which has two efficiency peaks on the graph and is more efficient at 105kph than at 72kph.


Here is the full wiki page: http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles

Sorry, but according to your graph it's opposide. Celica is tiny bit more efficient at 72km/h than 105km/h.

CiniO

Wetbench4 wrote: »

Yes i agree my 100/120kph examples were bad because of drag but at lower speeds 80/100kph it makes sense.

Indeed some vehicles out of this graph, seems to be more economical at speeds 90-95 km/h than at 70-80km/h.

Wetbench4

CiniO wrote: »

Sorry, but according to your graph it's opposide. Celica is tiny bit more efficient at 72km/h than 105km/h.

No its mpg, the higher the number the better. Are you looking at the right one, its the light blue line at the top.

edit: although id say its american gallons but im not sure.

And i see what your saying now, it should 60mph v 40mph

CiniO

Wetbench4 wrote: »

No its mpg, the higher the number the better. Are you looking at the right one, its the light blue line at the top.

edit: although id say its american gallons but im not sure.

And i see what your saying now, it should 60mph v 40mph

Ahh that's right.
60mph seems to be more economical for that celica than 40mph.
But 105km/h (65mph) is not more economical than 72km/h (45mph), as least according to the graph.

Wetbench4

CiniO wrote: »

Ahh that's right.
60mph seems to be more economical for that celica than 40mph.
But 105km/h (65mph) is not more economical than 72km/h (45mph), as least according to the graph.

Ya i didn't spot that, i just took the numbers from the article. Its interesting stuff though. The jeep cherokee is only about 6mpg lower at 70 than at 40.

Seweryn

Zubeneschamali wrote: »

No, the OP is asking what speed to drive for the best MPG, they're not asking to balance mpg against time.

That is what I got as well.

And there is no family type car out there that uses less fuel at 90km/h that it does at 70km/h. Laws of physics are simple and if the car has 17 gears, just drop it to keep the engine speed at "cruising minimum".