:roll:Originally posted by: BrownTown
Well, driving slower obviously increases gas milage, the only question here is by how much. That being said, the amount of TIME saved by driving fast more than makes up for the extra money. If you make say 30$ an hour then saving an hour on a trip is worth alot more than saving a gallon of gas. Personally I drive as fast as I think I can get away with because TBH even with all the bitching about gas prices they are still pretty darn cheap when you get right down to it.
Originally posted by: soccerballtux
5 o'clock local news stations have picked this up, saying you can save something like 20% more money by driving 55 instead of 75.
I think somebody's doing some fuzzy maths. Perhaps they mean you burn fuel 20% slower. But you're also going 20% slower. And it takes 25% more time to get there.
In my old 88 Camry I could get 30.5-32MPG going 70 all day. I say it's a rogue attempt to get people to drive slower.
Originally posted by: KurskKnyaz
Originally posted by: soccerballtux
5 o'clock local news stations have picked this up, saying you can save something like 20% more money by driving 55 instead of 75.
I think somebody's doing some fuzzy maths. Perhaps they mean you burn fuel 20% slower. But you're also going 20% slower. And it takes 25% more time to get there.
In my old 88 Camry I could get 30.5-32MPG going 70 all day. I say it's a rogue attempt to get people to drive slower.
you people are ignoring momentum.
someone who drives 25 miles at 25mph will burn more gas then someone who hits 90mpg and then lets go of the throttle.
Originally posted by: bobsmith1492
There is a maximum efficiency band for each vehicle depending on its engine, transmission, weight, and aerodynamics.
Typically you should do between 45-60MPH for peak fuel efficiency.
Examples:
http://www.consumerenergycente...mages/speed_vs_mpg.gif
http://www.fsec.ucf.edu/En/med...s/Speed_vs_Mileage.JPG
http://www.theoildrum.com/uploads/12/mpg_vs_speed.jpg
LOL, a Metro doing 35
Here's a good page
After a bit of looking, it would seem most cars are optimized to drive at about 55MPH.
That said, I typically do about 60 on the freeway.
Originally posted by: bobsmith1492
Originally posted by: KurskKnyaz
Originally posted by: soccerballtux
5 o'clock local news stations have picked this up, saying you can save something like 20% more money by driving 55 instead of 75.
I think somebody's doing some fuzzy maths. Perhaps they mean you burn fuel 20% slower. But you're also going 20% slower. And it takes 25% more time to get there.
In my old 88 Camry I could get 30.5-32MPG going 70 all day. I say it's a rogue attempt to get people to drive slower.
you people are ignoring momentum.
someone who drives 25 miles at 25mph will burn more gas then someone who hits 90mpg and then lets go of the throttle.
Please look at the graphs. You are wrong.
Originally posted by: bobsmith1492
There is a maximum efficiency band for each vehicle depending on its engine, transmission, weight, and aerodynamics.
Typically you should do between 45-60MPH for peak fuel efficiency.
Examples:
http://www.consumerenergycente...mages/speed_vs_mpg.gif
http://www.fsec.ucf.edu/En/med...s/Speed_vs_Mileage.JPG
http://www.theoildrum.com/uploads/12/mpg_vs_speed.jpg
LOL, a Metro doing 35
Here's a good page
After a bit of looking, it would seem most cars are optimized to drive at about 55MPH.
That said, I typically do about 60 on the freeway.
Originally posted by: Peter
No he won't, and ironically enough that's exactly because of momentum. Accelerating a given weight to 90 burns much more fuel than accelerating to 25, and once you've reached 90, the non-linearity of air drag will brake you back down much more intensely than if you were cruising at 25.
Being in the land of (somewhat) unlimited highway speeds and fuel efficient cars, I can report one thing: Driving slower IS cheaper - as long as you don't drop off the RPM range in which your car's engine is efficient. From 120 km/h to 200 km/h, fuel consumption about doubles. Dropping back to truck speed (80 officially, more like 90 to 100 in reality) still has noticeable effect.
This isn't only due to pure physics, but also because traffic is much more of a disturbance when you're going fast. If you're trying to do 180 even in mild traffic, you'll spend a lot of time braking and accelerating. When you're going at everyone else's speed, you'll be cruising along steadily and often in someone else's slipstream. When you're driving in the outside lane inbetween trucks, you're in a big fat slipstream, and fuel consumption is instantly down a third from driving the same speed in still air.
Originally posted by: Peter
No he won't, and ironically enough that's exactly because of momentum. Accelerating a given weight to 90 burns much more fuel than accelerating to 25, and once you've reached 90, the non-linearity of air drag will brake you back down much more intensely than if you were cruising at 25.
Being in the land of (somewhat) unlimited highway speeds and fuel efficient cars, I can report one thing: Driving slower IS cheaper - as long as you don't drop off the RPM range in which your car's engine is efficient. From 120 km/h to 200 km/h, fuel consumption about doubles. Dropping back to truck speed (80 officially, more like 90 to 100 in reality) still has noticeable effect.
This isn't only due to pure physics, but also because traffic is much more of a disturbance when you're going fast. If you're trying to do 180 even in mild traffic, you'll spend a lot of time braking and accelerating. When you're going at everyone else's speed, you'll be cruising along steadily and often in someone else's slipstream. When you're driving in the outside lane inbetween trucks, you're in a big fat slipstream, and fuel consumption is instantly down a third from driving the same speed in still air.
Generalizations like this one are useless. CC will not always adversely affect mileage. In fact, in many cases (probably most) it will improve mileage by decreasing unnecessary throttling. Simply adjusting the controller gains can easily allow for hill climbs and so on without penalizing mileage whatsoever. I know - I've seen me do it.Originally posted by: RideFree
Also, it should be noted that setting the cruise control will adversely affect your mileage.
The CC always tries to maintain set speed, even going uphill.
This is a no, no for good mileage.
Engineers tune cruise controls for various purposes during the design and testing of every car. Whether they tune it for mileage or performance depends on the goals of the design team. I adjusted mine on my old car with a screwdriver.Originally posted by: RideFree
My cruise control always gives it more gas going up hill.
It is better to maintain the "egg" between your foot and the gas pedal and keep 'er steady
than to give extra gas in order not to lose any speed on an uphill slope.
Assuming, of course, we are talking about someone who knows how to drive for mileage.
Across Nevada (in fact, west of Salt Lake City), the CC would probably make little difference, as it's flatter than a pancake.
Across Kansas, I win the MPG race.
PS I'd like to know the specifics of how you adjusted those controller gains.
And, how does this qualify as a generalization when in fact,
you are the only person that I've heard of who did this "controller gain adjustment"?
Originally posted by: Peter
It's not as easy as you think ... simply because engines, modern supercharged engines in particular, have a pretty narrow RPM band in which they're good and efficient. This is why gearboxes tend to have more and more gears, even automatic gearboxes these days have up to seven. Guess why.
So where's the sweet spot then? Somewhere in the triangle between engine efficiency, air drag, and gearbox ratios.
With those 7-speed gearboxes and a turbocharged diesel with its typical narrow efficiency band, the sweet spot may not even be in the highest gear - at least not with gearboxes for Europe geared for high max speed.
Regarding cruise control: Every change in throttle costs extra, particularly in petrol engines, much less so in diesels. So if your CC desperately tries to keep an exactly steady speed and adjusts the throttle all the time, it'll do worse than a steady-footed human who doesn't care whether he's doing 99.4 or 100.3.
On the other hand, putting the right foot further down to go uphill is the right thing to do - if this move keeps the engine inside its efficiency window.
Peter
PS: Could you please sort your it's and its? You're confusing us non-native readers ... thanks.
What if my car is a Prius (which, as of yesterday, it is) and has a continuously variable transmission?Originally posted by: superunknown98
I don't understand why everyone is so hung up on "speed". This has nothing to do with efficiency by itself. Efficiency is a product of the car: gearing, weight, areodynamics, transmission, engine torque. A car will be most efficient in it's highest gear at it's lowest rpm. Obviously this is not a real world driving conditon, as hills exist. However in a manual car if you idle in 5th or 6th gear, you will probably be going 40 to 55MPH depending on your gearing. The higher the RPM the more gas your engine will burn, end of story. Engine efficiency is how much gas is needed to make a certain amount of torque. just beacause your car makes the most torque at 5000RPM does not mean it is more economical at that RPM. That is it's most efficient RPM in terms of power and it can make more power at that RPM given a certain amount of fuel than any other RPM. Other than the mechanicals of the car, there is areodynamics to worry about. The faster you go the more air your car has to push. Think about trying to run in a swimming pool. Walking is easy in water, but the faster you try to run the harder it is.
Just becasue you drive faster and arrive sooner does not negate the fact that your car had to burn extra fuel to provide the extra power it needed to move the air. your car may be turned off 15 early but your car had to stay at 2.5 times the normal RPM's.
Speed is a product of the engines power at a given RPM and gear minus air friction. It's why you can't say all cars get the best MPG at 60MPH. And please don't say your car get's its best milage at a certain RPM, becasue the best economy will always be at the lowest amount of power required to maintain speed in any gear.
I only brought this up because it demonstrates the non-universality of the "low RPMs FTW!" approach. The energy balance approach I stated above is completely universal, though obviously harder to follow since all of the terms vary for each car and situation in a way that is not obvious.Originally posted by: AnnonUSA
CVT's are supposed to keep ratios optimal, but because of driveability issues they too must compromise. There is no free lunch.
Hear! Hear!Originally posted by: AnnonUSA
Keep in mind that Hypermileing, while it may save the tards that do it fuel, waste more fuel overall than just driving more sensibly. Starting up from stop lights at a snails pace causes less traffic to get through the light each cycle, causing more traffic, more idling cars, wasting gas and causing pollution. Slowly merging onto highways at sub freeway speeds, causes other traffic to have to slow down, and brake, wasting momentum and gas, and causing traffic that (see example #1).