Driving slower = better gas mileage. Fact or myth?

[Peter]

Your main argument has been negated multiple times already in this thread. See a bit further up, the paragraph in bold.

Driving slow is dangerous, yes, but so is 'fast'. Readjust your sense for 'reasonable' speed and acceleration, learn to drive more steadily, and there's your MPG reduction with ADDED safety.

 

[Xecuter]

It does help MPG. I used to drive This aerodynamic mess at 75MPH and would get around 22MPG. I just drove a 1000 mile round trip at 55MPH and got 30MPG.

 

[CycloWizard]

Originally posted by: Xecuter
It does help MPG for my car under the conditions I drove under on this trip and slowing from speed x to speed y.

Fixed.

 

[KIAman]

Throwing a wrench in all of this. Altitude and turbo/super charging.

 

[Net]

every car's optimal speed will be different but overall that number is probably pretty close.

you want to stay at the same speed that will maximize low engine usage vs speed.

riding your bike. once you hit a certain speed all you need is a few peddles to maintain it. you end up getting to your target with less work done. less work done for you means less energy you waste. now for the car its less gas used.

 

[cusideabelincoln]

There are many factors, but overall it is a fact. Driving at 55 as opposed to 70 has these benefits:

-Most transmissions and engines are built to go into Overdrive somewhere between 50-60, like previously stated.
-This means the engine running at it's lowest RPMs while the car is going it's fastest speed, between 50-60. My car seems to shift into OD at around 48 MPH.
-Lower engine RPMs means there is much less internal friction between the piston rings and cylinder walls and and between various bearings and shafts.
-Lower cruising speeds also means less air drag, which would vary greatly from vehicle to vehicle.

Of course there are other factors to consider:
-How quickly you accelerate.
-Air density, temperature, and oxygen content.
-Vehicle weight.
-Drafting.

Yes, I definitely notice an improvement in my gas mileage whenever I draft off of semi trucks on highway trips. The improvement I've noticed is usually about 3 MPG.

 

[EricMartello]

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 can reach your peak fuel efficiency based on engine speed in your highest gear...so if you have a 5-speed, the 5th gear is overdrive and if you have a 4-cyl car you'd probably find keeping the engine at a steady 2,200-2,800 RPM will give you the most MPG. The ideal RPM varies based on your car's actual gear ratios...but usually that speed works out to 65-70 MPH (they're not gonna tell you to exceed the speed limit on a news report - most places still have 55 MPH as the max speed).

Steady highway driving does not impact fuel efficiency nearly as much as hard acceleration. Gentle acceleration will net you better returns on fuel economy than anything else in terms of driving style.

 

[supremelaw]

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[Tary88]

My first post woohoo!

A engineer from ford married into my family about 6 or 7 years back now. I asked him what role speed played in regards to gas mileage. He told me the entire line-up from top to bottom (including the trucks) are engineered for best mileage at 80 km/hr. Anything over or under burns more fuel. Obviously there are alot of factors that come into play, but thats the sweet spot for normal driving conditions.

 

[KIAman]

Originally posted by: Tary88
My first post woohoo!

A engineer from ford married into my family about 6 or 7 years back now. I asked him what role speed played in regards to gas mileage. He told me the entire line-up from top to bottom (including the trucks) are engineered for best mileage at 80 km/hr. Anything over or under burns more fuel. Obviously there are alot of factors that come into play, but thats the sweet spot for normal driving conditions.

I might believe that for trucks but I have a hard time believing that engineers would purposely tune their cars and small cars to be most efficient at 50MPH. A lot of cars will bog down at that speed in top gear. Not only does it go against engine/transmission mechanics but also not optimal for most of the world's highway speeds.

If this is true, then they lacked the vision to produce gas efficient cars for today's drivers and I understand why they are losing so much money.

 

[PieIsAwesome]

Ok, here is a question.

On level ground, in a vacuum, what would a car have to overcome after having accelerated to a desired speed? What would be the reason, if any, to continue burning fuel?

Friction in all moving parts of the car comes to mind, but how much of an effect does it have, and what else besides that?

If no air resistance means no reason to continue burning fuel, and assuming friction in moving parts is negligible, then can we assume that air resistance is the reason for continued fuel use and that reducing air resistance will reduce fuel consumption? If reducing speed reduces air resistance then reducing speed reduces fuel consumption?

 

[CycloWizard]

Originally posted by: PieIsAwesome
Ok, here is a question.

On level ground, in a vacuum, what would a car have to overcome after having accelerated to a desired speed? What would be the reason, if any, to continue burning fuel?

Friction in all moving parts of the car comes to mind, but how much of an effect does it have, and what else besides that?

If no air resistance means no reason to continue burning fuel, and assuming friction in moving parts is negligible, then can we assume that air resistance is the reason for continued fuel use and that reducing air resistance will reduce fuel consumption? If reducing speed reduces air resistance then reducing speed reduces fuel consumption?

Internal friction and rolling friction (tires on pavement). At speed, the work otuput of the engine must match the energy draw of these two sinks.

 

[theMan]

my car has instantaneous MPG on the dash. when i drive at a constant 65mph on a level highway, i get about 27mpg. when i drive about 80mph (like most people here in MI) it goes down to around 23-24mpg.

another factor to look at besides wind resistance is engine RPM. the engine is more efficient at lower RPMs so if you are going 80 and your car has to do about 2800rpm it's less efficient than if you're going 65 and your car only has to run at 2100rpm or so.

And, the best way to get better gas milage is to pretend you're on a bike. when you're cycling, you want to really pedal fast going down hills, so its easier to go up hills because you have all the momentum already. so, when i drive, i push my foot down ever so slightly on down hills, so my speed increases about 10mph, and when i go up hills, i let my foot off a bit, or even take it completely off the gas depending on how steep or long the hill is. this works extremely well. its just important to remember to keep those RPMs down. you never want to push the pedal down more than a little bit. keep everything smooth. my city MPG average has gone up about 6 since i started doing that.

 

[KIAman]

Originally posted by: theman
my car has instantaneous MPG on the dash. when i drive at a constant 65mph on a level highway, i get about 27mpg. when i drive about 80mph (like most people here in MI) it goes down to around 23-24mpg.

another factor to look at besides wind resistance is engine RPM. the engine is more efficient at lower RPMs so if you are going 80 and your car has to do about 2800rpm it's less efficient than if you're going 65 and your car only has to run at 2100rpm or so...

Your instant MPG gauge is still a mathematical formula, so not 100% accurate but a pretty good estimate. It counts the number of injector pulses and multiplies that by the injector volume.

RPM is not the only factor for engine efficiency. There are other factors such as compression ratio, the length of time intake/exhaust gasses stay in the combustion chamber (valve timing), the amount of intake air, the amount of intake gas, rotational mass (counter-weights and flywheel), exhaust backpressure, gearing, the number of cylinders active, etc.

There is no true rule of thumb other than it would be logical for car manufacturers to set optimal efficiency at average world freeway speeds (to apply to the most people possible). In Europe, you got the autobahn...

 

[Peter]

... and standardized fuel efficiency and emission tests that include speeds up to 130 km/h.

However, we don't drive anywhere near as much on highways as you people in the US do. Europe is densely populated, we don't (and don't need to) drive around for hours to get stuff done.

 

[Uhtrinity]

My Car, straight line, flat ground, no starts or stops, no electric assist or load, steady speed, average summer day.

25 mph - 100 mpg
35 mph - 120 mpg
45 mph - 100 mpg
55 mph - 85 mpg
65 mph - 80 mpg
70 mph - 75 mpg
75 mph - 65 mpg
80 mph - 60 mpg
90 mph - 50 mpg

Very repeatable within +/- 5 mpg based on the realtime mpg gauge. The Gauge itself as well as the mpg trip meter have shown to be within a few mpg of calculations at fillup. Keeping rpms under 3k at highway speeds nets far better operation at highway speeds than rpms over the 3500 range. The difference being 2 valves per cylinder operation vs 4 at higher rpms.

 

[SJP0tato]

Originally posted by: CycloWizard

Originally posted by: PieIsAwesome
Ok, here is a question.

On level ground, in a vacuum, what would a car have to overcome after having accelerated to a desired speed? What would be the reason, if any, to continue burning fuel?

Friction in all moving parts of the car comes to mind, but how much of an effect does it have, and what else besides that?

If no air resistance means no reason to continue burning fuel, and assuming friction in moving parts is negligible, then can we assume that air resistance is the reason for continued fuel use and that reducing air resistance will reduce fuel consumption? If reducing speed reduces air resistance then reducing speed reduces fuel consumption?

Internal friction and rolling friction (tires on pavement). At speed, the work otuput of the engine must match the energy draw of these two sinks.

Additionally there's the friction of the engine, transmission, drivetrain, and accessories (alternator, power steering, various pumps). I've read/heard figures that something like 10-15 horsepower is lost in friction and internal pumping in the engine alone in modern cars. So a 4-cylinder rated at 100hp would put out 110-115hp in a completely frictionless world.

Also there is a 12-18% power loss through the transmission and drivetrain. Engine dynos read 12-18% higher than wheel dynos when tested. Things like straight-cut gears within the transmission can help, but there's still losses.

With all this going on in an air friction-less environment, it makes you wonder if you'd be better off getting up to speed then coasting a fair distance before speeding back up, or just keeping the engine running full-time to hold a steady speed.

 

[CycloWizard]

Originally posted by: SJP0tato
Additionally there's the friction of the engine, transmission, drivetrain, and accessories (alternator, power steering, various pumps). I've read/heard figures that something like 10-15 horsepower is lost in friction and internal pumping in the engine alone in modern cars. So a 4-cylinder rated at 100hp would put out 110-115hp in a completely frictionless world.

Also there is a 12-18% power loss through the transmission and drivetrain. Engine dynos read 12-18% higher than wheel dynos when tested. Things like straight-cut gears within the transmission can help, but there's still losses.

With all this going on in an air friction-less environment, it makes you wonder if you'd be better off getting up to speed then coasting a fair distance before speeding back up, or just keeping the engine running full-time to hold a steady speed.

Yeah, those are what I called "internal friction." I have also had success coasting up hills and accelerating down. My mileage has improved considerably using this approach, though I'm pretty sure it drives everyone else on the road insane. I also got nearly 10 mpg more driving through northern Indiana, which is as flat as a pancake.

 

[Smilin]

Toyota Prius vs BMW M3 (V-8).

http://www.youtube.com/watch?v=0o63BOVlzhc


The BMW wins by a couple MPG when the Prius is going full throttle.

 

[Peter]

Of course it does. The Hybrid system doesn't help there, and at its top speed, the Prius is well beyond its efficiency window - while the BMW is still in its efficient midrange RPMs, and brings aerodynamics for much higher speeds than that.

If you want to run that kind of comparison at a much lower speed, just look at how inefficient a Smart at its top speed (about 140 km/h) is - and then look at how miserably that V8 BMW does in city traffic.

Stuff built for different purposes has different efficiency windows. Did that surprise anyone?