Driving slower = better gas mileage. Fact or myth?

[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.

 

[bsobel]

Air resistance isn't linear.

 

[soccerballtux]

I would argue for the speeds we're talking about, <150kmph, it largely is.

 

[TuxDave]

Originally posted by: soccerballtux
I would argue for the speeds we're talking about, <150kmph, it largely is.

And where did your 150km/h number come from?

And as much as I hate linking to wikipedia.... I'm too f'ing lazy today so you can go read up on this drag equation to see the non-linear relationship.

http://en.wikipedia.org/wiki/Drag_equation

 

[lousydood]

Faulty reasoning or not, this was used to justify lowering the natl highway speed limit to 55 back in the 70s.

 

[dkozloski]

Many modern cars have a dashboard computer readout that among other things displays instantaneous miles per gallon. On level ground, my '06 Cadillac AWD V8 STS clearly shows that gas milage increases at lower speeds down to about 35MPH. Above 60 MPH it drops rapidly. Rolling resistance as well as aerodynamic drag increases with speed.

 

[AnnonUSA]

With a decently engineered car (not SUV) fuel efficiency can be dialed in via gearing, and aero modifications.

While speed does play a part, most average drivers are not smart enough to save gas with their right foot. I had an Acura TL-s that would get great mileage on the highway. The trans had the manual mode lever, and once on a trip from Florida to NJ I got almost 33 miles a gallon (Average speed 75MPH, all highway, trans locked in 5th gear, and mixture of foot and cruise control).

I would agree with one of the prior posts, time is money too, and for a long interstate trip, doing 80 or 90 instead of 55 can subtract hours of windshield time, eliminating driver fatigue, and at a minimal cost disadvantage.

It won't be long before Congress once again starts looking at altering Speeds to "save" gas. Cause as we all know they will never do anything concrete about the true cause of the Oil pricing. All we can expect is lower speed limits (Which saves minimal amounts of gas, but produces revenue via fines), asinine regulations, and feel good bullshit like Gas Tax "holidays".

 

[KIAman]

Were you the one who asked this question?!?

http://www.cartest.ca/what_speed_should_i_drive_to.htm

 

[bsobel]

Originally posted by: soccerballtux
I would argue for the speeds we're talking about, <150kmph, it largely is.

You can argue all you like, your still wrong.

 

[scruffypup]

driving slower will save gas, which in turn saves actual money. The main reasoning is due to engine efficiency,... an engine will become less efficient when spinning at higher rpms. This is why gearing is set to have a car running typically around 2000 to 2800 rpm range (all vehicles will vary) when cruising at 65mph so that the engine is more efficient. Otherwise you would have the engine running 5000+rpms and your mileage would be going out the exhaust quite literally,... some percentage of unburnt gas and so on. One way to combat this would be to have even higher gearing for speeds in excess of 75 mph, but there are reasons this is not done,.. more social and economic than not being able to.

 

[CycloWizard]

Ah, I was wondering when this debate would wind up here. First, I'll say that I kept mileage logs for my old 1990 Acura Integra for 11 years until about three months ago when she gave up the fight. I got the highest mileage when trying to maintain a speed of 82 mph. Above that speed, it dropped off fairly quickly (though I didn't have much data up there ), this is likely due to a number of factors, especially having to slow down for traffic.

Originally posted by: TuxDave
And as much as I hate linking to wikipedia.... I'm too f'ing lazy today so you can go read up on this drag equation to see the non-linear relationship.

http://en.wikipedia.org/wiki/Drag_equation

There are a few assumptions that go into that formulation that limit its applicability to this discussion. The Rayleigh equation gives the drag on a particle in an infinite medium. Obviously, this doesn't really hold up for a car, since there are always interactions between the bottom (and even sides) of the car and the road. Further, it only holds at asymptotically high Reynolds numbers (i.e. viscous effects are completely negligible and flow is fully turbulent), which is incorrect for nicely designed cars. A lot of car manufacturers have spent a great deal of time treating the bottom of the car to reduce drag. If it were as simple as applying this equation, wind tunnels would barely be used. I'll add some more later.

 

[Crusty]

Don't forget taht driving slower means it takes you longer to get somewhere!

 

[BassBomb]

Originally posted by: Crusty
Don't forget taht driving slower means it takes you longer to get somewhere!

It depends. I drive slower home now, I get there same time. Drive slower on the way to work it takes longer.

 

[Crusty]

Originally posted by: BassBomb

Originally posted by: Crusty
Don't forget taht driving slower means it takes you longer to get somewhere!

It depends. I drive slower home now, I get there same time. Drive slower on the way to work it takes longer.

Of course it depends, depends on the kind of road, type of traffic, weather conditions, make of the car, type of gas you use, how you accelerate, etc etc

 

[herm0016]

i have started driving just a few mph slower on long trips, i usuallygot around 30mpg. my last trip to green bay was 32mpg. it does help. I drive a 99' oldsmobile eightyeight with a 3.8L V6 too.

 

[KIAman]

To all the people who keep throwing in that driving slower takes longer to reach your destination... the metrics of saving gas is in MILES PER GALLON. If your MPG goes up slower, then it is irrelevant that it took longer to get there, you WILL be saving gas.

Also, read the link that I made earlier, it has a great description of gas consumption based on speed.

Snippet

In effect the efficiency of the engine is improving. It uses a fixed amount of fuel to power itself and the accessories, and a variable amount of fuel depending on the power required to keep the car going at a given speed. So in terms of fuel used per mile, the faster the car goes, the better use we make of that fixed amount of fuel required.

This trend continues to a point. Eventually, that road load curve catches up with us. Once the speed gets up into the 40 mph range each 1 mph increase in speed represents a significant increase in power required. Eventually, the power required increases more than the efficiency of the engine improves. At this point the mileage starts dropping. Let's plug some speeds into our equation and see how a 1 mph increase from 2 to 3 mph compares with a 1 mph increase from 50 to 51 mph. To make things easy we'll assume a, b and c are all equal to 1.

Speed

Equation

3 mph | 3+3^2+3^3 | 39
2 mph | 2+2^2+2^3 | 14
Power Increase = 25

51 mph | 51+51^2+51^3 | 135,303
50 mph | 50+50^2+50^3 | 127,550
Power Increase = 7,753

You can see that the increase in power required to go from 50 to 51 mph is much greater than to go from 2 to 3 mph.

So, for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph. Cars with a higher road load will reach the sweet spot at a lower speed. Some of the main factors that determine the road load of the car are:

* Coefficient of drag. This is an indicator of how aerodynamic a car is due only to its shape. The most aerodynamic cars today have a drag coefficient that is about half that of some pickups and SUVs.
* Frontal area. This depends mostly on the size of the car. Big SUVs have more than double the frontal area of some small cars.
* Weight. This affects the amount of drag the tires put on the car. Big SUVs can weigh two to three times what the smallest cars weigh.

In general, smaller, lighter, more aerodynamic cars will get their best mileage at higher speeds. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds.

If you drive your car in the "sweet spot" you will get the best possible mileage for that car. If you go faster or slower, the mileage will get worse, but the closer you drive to the sweet spot the better mileage you will get.

 

[TuxDave]

Originally posted by: CycloWizard
Ah, I was wondering when this debate would wind up here. First, I'll say that I kept mileage logs for my old 1990 Acura Integra for 11 years until about three months ago when she gave up the fight. I got the highest mileage when trying to maintain a speed of 82 mph. Above that speed, it dropped off fairly quickly (though I didn't have much data up there ), this is likely due to a number of factors, especially having to slow down for traffic.

Originally posted by: TuxDave
And as much as I hate linking to wikipedia.... I'm too f'ing lazy today so you can go read up on this drag equation to see the non-linear relationship.

http://en.wikipedia.org/wiki/Drag_equation

There are a few assumptions that go into that formulation that limit its applicability to this discussion. The Rayleigh equation gives the drag on a particle in an infinite medium. Obviously, this doesn't really hold up for a car, since there are always interactions between the bottom (and even sides) of the car and the road. Further, it only holds at asymptotically high Reynolds numbers (i.e. viscous effects are completely negligible and flow is fully turbulent), which is incorrect for nicely designed cars. A lot of car manufacturers have spent a great deal of time treating the bottom of the car to reduce drag. If it were as simple as applying this equation, wind tunnels would barely be used. I'll add some more later.

First imagine the cow is a sphere......

 

[Crusty]

Originally posted by: KIAman
To all the people who keep throwing in that driving slower takes longer to reach your destination... the metrics of saving gas is in MILES PER GALLON. If your MPG goes up slower, then it is irrelevant that it took longer to get there, you WILL be saving gas.

Also, read the link that I made earlier, it has a great description of gas consumption based on speed.

Snippet

In effect the efficiency of the engine is improving. It uses a fixed amount of fuel to power itself and the accessories, and a variable amount of fuel depending on the power required to keep the car going at a given speed. So in terms of fuel used per mile, the faster the car goes, the better use we make of that fixed amount of fuel required.

This trend continues to a point. Eventually, that road load curve catches up with us. Once the speed gets up into the 40 mph range each 1 mph increase in speed represents a significant increase in power required. Eventually, the power required increases more than the efficiency of the engine improves. At this point the mileage starts dropping. Let's plug some speeds into our equation and see how a 1 mph increase from 2 to 3 mph compares with a 1 mph increase from 50 to 51 mph. To make things easy we'll assume a, b and c are all equal to 1.

Speed

Equation

3 mph | 3+3^2+3^3 | 39
2 mph | 2+2^2+2^3 | 14
Power Increase = 25

51 mph | 51+51^2+51^3 | 135,303
50 mph | 50+50^2+50^3 | 127,550
Power Increase = 7,753

You can see that the increase in power required to go from 50 to 51 mph is much greater than to go from 2 to 3 mph.

So, for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph. Cars with a higher road load will reach the sweet spot at a lower speed. Some of the main factors that determine the road load of the car are:

* Coefficient of drag. This is an indicator of how aerodynamic a car is due only to its shape. The most aerodynamic cars today have a drag coefficient that is about half that of some pickups and SUVs.
* Frontal area. This depends mostly on the size of the car. Big SUVs have more than double the frontal area of some small cars.
* Weight. This affects the amount of drag the tires put on the car. Big SUVs can weigh two to three times what the smallest cars weigh.

In general, smaller, lighter, more aerodynamic cars will get their best mileage at higher speeds. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds.

If you drive your car in the "sweet spot" you will get the best possible mileage for that car. If you go faster or slower, the mileage will get worse, but the closer you drive to the sweet spot the better mileage you will get.

You are assuming that peoples time isn't worth anything. If I leave at the same time, and drive half the speed it's going to take me twice as long to get there(assuming everything else is constant). Now if I'm headed to work, I'd gladly waste more gas to get there earlier to put myself on the clock to earn more money. Since most people drive to and from work you can not ignore the fact that you are wasting that extra time when you could in fact be working and making money to offset the cost of the extra gas.

Knowing your MPG is hardly painting the whole picture, you still need to know how far you drove and how long it took you.

 

[CycloWizard]

Originally posted by: TuxDave
First imagine the cow is a sphere......

If a spherical cow is falling through a viscous liquid, then Stokes-Einstein applies and the problem is completely different. We had this discussion in P&N a while ago and no one wanted to believe that the Rayleigh equation didn't govern drag forces under all conditions. That's why I'm glad we're finally talking about this in HT.

Originally posted by: lousydood
Faulty reasoning or not, this was used to justify lowering the natl highway speed limit to 55 back in the 70s.

At the time, it made sense because of the way cars were designed. SOHC (single overhead cam) engines gave a relatively sharp efficiency peak. My car (a '90) was DOHC (dual OHC), so the ability to broaden the efficiency peak has been around at least since then and probably a lot longer, though I'm hardly a big car history buff to be able to tell you exactly when. The P&N discussion came up because people were suggesting implementing a national cap of 60 mph, which just doesn't make sense. A one-size-fits-all approach to a very complicated problem is bound to cause at least as many problems as it solves and could very well result in us burning more, rather than less, gas.

 

[lousydood]

Cars at the time I imagine were mostly American made and with a pushrod design. Those were not well suited to high revs like the other posted suggested. The overhead cam design of many modern engines is much better at high RPMs. It's actually encouraged in european cars like mine to hit 5-6k RPMs, the engine is designed to work well at those ranges. I find my car has a pretty flat torque curve, which is nice for the low-end, and it spikes a little around 4k. It gets 28 mpg road which was pretty good for its class and model year. Of course sticking to higher gears is going to help a bit with the fuel efficiency.

Sadly, one-size-fits-all solutions are popular with the politics crowd. Simplistic solutions for simplistic minds. As others have pointed out, my time is worth more than the gas, even now, and if I want to spend the money I should be able to waste the gas. I'm looking at a 10 hour road-trip next week and the gas prices do not bother me, it's the loss of a day and hours of stress dealing with traffic (stupid <insert state> drivers!), construction, greedy cops, fatigue, etc. Driving feels very much like work to me now, not fun, which sucks.

 

[KIAman]

Quick googling shows in 2005, the average American commute to work is 16 miles, so 32 miles for roundtrip.

Let's assume complete freeway driving at a constant speed (to keep the calculation simple).

75MPH going 32 miles = 25.6 minutes travel time.
Avg mileage for 75MPH = 27 going 32 miles = 1.185 gallons spent = 5.33 dollars per day

65MPH going 32 miles = 29.5 minutes travel time.
Avg mileage for 65mph = 30 going 32 miles = 1.067 gallons spent = 4.80 dollars per day

Time saved: roughly 4 minutes per day
Dollars spent: $0.53 extra per day

Assuming you get paid by the minute, the minimum hourly wage you need to earn to offset the cost is $7.95 an hour.


Now, let's assume the average car being a non compact car.

75MPH going 32 miles = 25.6 minutes travel time.
Avg mileage for 75MPH = 15 going 32 miles = 2.133 gallons spent = 9.60 dollars per day

65MPH going 32 miles = 29.5 minutes travel time.
Avg mileage for 65mph = 20 going 32 miles = 1.6 gallons spent = 7.20 dollars per day

Time saved: roughly 4 minutes per day
Dollars spent: $2.40 extra per day

Assuming you get paid by the minute, the minimum hourly wage you need to earn to offset the cost is $36.00 an hour.

 

[AnnonUSA]

I remember the 70's Gas Crisis, we were told the world's supply of crude was at best enough for about 10 more years. We were lied to then, we are being lied to now. Different lies, different reasons but lies none the less.

The bottom line is a skilled driver can outperform most average drivers in fuel mileage, and not have to sacrifice much in the way of performance. Most people cannot short shift an Automatic Trans with their right foot, or use an engine, gearbox and brakes to their potential without wasting gas. Most would get better gas mileage with a manual trans too, but the delta between Auto and Manual has been reduced over the years for average drivers.

 

[CycloWizard]

To look at the original question a little more closely, I suppose we really need an energy balance. W=work, n=efficiency.

W_engine=W_output

W_engine=(combustion energy of gasoline)*(overall efficiency of the engine)

The combustion energy is dependent on the gasoline formulation, as well as the operating temperature and pressure (weak pressure dependence, strong temperature dependence).

The overall efficiency of the engine depends on the engine design, transmission (gearing and other things), operating temperature, ambient temperature, and frequency (rpms).

W_output=W_drag+W_(rolling friction)+W_acceleration

W_drag and W_rf are functions of speed. W_acceleration is easily computed from Newton's second law (W=F*d=(m*a)*d in algebraic form... can't really write the proper calculus-based form here).

Which terms are influenced by how you drive? Engine efficiency and all the work terms. Which terms are influenced by the car's design? Same thing. The formulation for the work required to overcome drag is very complicated, as is the engine efficiency. However, with those in hand, one could actually compute the ideal parameters to maximize fuel efficiency. You'd just need a very powerful computer to do it, otherwise you'd be waiting for days/weeks for the solution (like I'm doing now for a much less complex problem ).

 

[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.

 

[RideFree]

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.

Shades of Nixon!