Truck and Barter

Driving fast imposes huge negative externalities on others. Tens of thousands of Americans are basically murdered by negligent drivers each year, yet we continue to call these homicides 'accidents.'

Perhaps black boxes in cars and 'smart road' technology will improve commuting. If driving speed and lane changes could be precisely controlled in key places at key times, perhaps road capacity would increase, while commute times and traffic fatalities decline. Pehaps 'smart lanes' could be built as toll lanes so the public would see them as a new choice and not an imposition.

In my car, on long interstate trips, I see no significant differences in mileage between going 70 and going 95. If anything, it may have been better.

I suspect that there is a *lot* of variability in different cars depending on engine size and torque, aerodynamics, and especially gearing. Perhaps the elevation at which I was driving also made a difference, as the higher-speed driving tended to be in the high west. Hmm. Maybe the air-conditioning was the main factor.

"Stop speeding. The faster you drive, the more gas you use, the more money you spend. Each 5 mph over 60 is equivalent to paying an extra 10 cents per gallon for gas...."

This is nonsense.

If you pay $2.11 for a gallon of gas, you've paid $2.11 for a gallon of gas, whether you burn that gallon travelling ten miles or thirty miles.

You can save pennies per mile, but not pennies per gallon.

Okay, she's got the units wrong (you are paying more per mile, not more per gallon by speeding).

HOWEVER...

The physics are undeniable. Driving faster increases the drag force due to aerodynamics on your vehicle. The drag force increases at the square of your velocity, so driving twice as fast results in 4 times the drag force.

You WILL get better gas mileage at 60 mph (or even better, 55 mph) than at 95 mph. There isn't even an argument there. But you do need to determine how much your time is worth, as well as other factors like safety, blood pressure, etc. During my commute, I generally pump up the tunes, set the cruise control at 65, and park myself in the center lane. It takes me longer to get home, but it is a lot less stressful.

I'm a mechanical engineer, by the way. So I know what the heck I'm talking about on this subject.

To what extent does variation in engine design and gearing affect the placement of the efficiency peak, though? Drag force can't be the only relevant variable.

On a related note, does anyone know

-- what the highway speed used in determining the EPA highway mileage figures is?

-- same for the European "extraurban cycle" figure?

It seems intuitively likely that the Euro-cycle would use a higher speed, since Europeans drive much faster on their highways; and the Euro extraurban figures I've seen tend to be higher than the EPA highway number for the same car with the same engine.

There isn't a car sold in America that isn't in top gear by 55 mph. In fact, most automakers tune their transmissions to get them into top gear as soon as possible in order to save gas.

I'm not saying that slowing down is going to save a HUGE amount of gas for every vehicle. Some cars are more aerodynamic than others. Of course, SUVs, trucks and minivans will save the most money by slowing down.

But gas WILL be saved by slowing down in ANY car. Again, it is physics.

The EPA highway mileage is done at 55 mph. This is a huge bone of contention now that speed limits have been deregulated. It would be more realistic to do the test at 65mph, but that would lower mileage ratings and CAFE numbers. Car makers would have to sell more small cars, not a good thing for the bottom line.

Any car with a 6-speed can do 55 in 5th gear, and any car with a 5-speed can do it in 4th. In fact, a 6-speed Corvette can do 60 in *2nd* gear (hardly a typical example, just a striking data point).

The point is, Gear Ratios Matter. If an auto maker wanted, they could push the fuel economy peak to a higher speed. I would contend the demand really isn't there though...

Have you ever driven a 6 speed Corvette? It has a wicked annoying feature called a "skip shift". If you leave the stoplight in a leasurely manner, you can't shift from first to second (or third) gear. You have to go right to 4th gear!

This, of course, is a way to improve the 'Vette's mileage.

The 6 speed itself is a great way to save gas. The 'Vette is barely above idle at 55 mph in 6th gear. That is why the highway mileage of a 405hp Z06 'Vette still meets CAFE standards.

The point is that no one actually DRIVES a 'Vette that way. The point is that it CAN be driven that way during the EPA test, and the mileage that results meets CAFE standards.

Oh, and if $2.50 a gallon gas is getting you down, YOU can drive the 'Vette that way and get better mileage.

The fact that NO ONE is slowing down on the highway proves that, despite all the bitching, gas prices are not very high. If they were, people would slow down to get better mileage.

The best mileage will be achieved right at the point that the transmission gets into the highest gear and the torque convertor locks up (on an auto). This speed of course varies from car to car, but the ballpark is around 45mph.

The first comment is nonsense. Read this study. It shows that when they removed the national 55mph speed limit, fatalities went down.

http://www.safespeed.org.uk/speed-doesnt-kill.pdf

Just an addon. With my 98 escort zx2, JBA header, Pacesetter 2.25 exhaust, .405/242 intake cam, intake Cam gear set at +2, Stock ride hieght,stock computer, 3 inch Aluminum intake, 60mm TB, stock rims and tire size. I got 40mpg at 60mph, 70mph, 80mph, and 90mph but once I got above 93 my gas mileage went progessivly down to no lower than 33mpg which is still higher than the EPA tested 27mpg I'm supposed to get.

I wonder how many of you smart alecs are slowing down now ...?

FACT: The speed limit is the LAW. Obey it! The life you save could be mine.

"The fact that NO ONE is slowing down on the highway proves that, despite all the bitching, gas prices are not very high. If they were, people would slow down to get better mileage"

I have a 2003 Dodge Caravan with 6 cyl. I typically drive 60mph on 120 daily round trip to work. I was getting 24.7 mpg at 60 mph and am now getting 28 mpg (over last two tanks of gas) at 55 mph on the tollway and being VERY gentle on accelleration and braking when not. About 11% savings. If we are dumb enough to pay the high prices without doing something proactive, the oil companies will surely oblige us.

When you are in top gear, the only variable becomes drag. The amount of gas you use depends on the RPM of the engine (thusly, driving 35 mph in 2nd or 5th in a manual will produce a dramatic effect on mileage [and engine life...]), which then goes through the transmission (the only variable reducer for engine speed), then to the axle ratio. From there, the wheels turn, and you move the diameter of the tire times pi (circumference) inches. Done many consecutive times quickly, these inches constitute a readable MPH.

As speed increases, the force of air slowing you down increases. What can happen as the speed increases to a point different for each car is that the engine is working enough so that it would be going 75 mph for examle, yet you're only going 70mph. I don't know what the actual effect of drag is, just that its there and thats how it would end up.

If you really want to find out the effect of drag, its kind of simple, really. The formula to find out how fast you SHOULD be going is as follows:

MPH = (Engine RPM * Tire Diameter * pi) / (Gear Ratio of gear for tranmission * Axle ration [final drive] * 1056)

The tire diameter is (the width of the tire in mm * the profile / 50)/25.4 + rim size = tire diameter in inches.

For example, my tires are 235/45R17. (235*45/50)/25.4 + 17 = 25.33 inches.

Given this, and a transmission that has the following gears I - 3.42 II - 2.14 III - 1.45 IV - 1.00 V - 0.70, and a final drive of 4.06, I can do a little test. (If this seems high to you, well, it kind of is, it can easily start in 2nd. BTW, I average about 22mpg city, 28 highway, and its a 170hp 165ft-lb V6 on a 2800lb car).

At 65mph in 5th, I should have 2451RPM. At 65mph in 3rd, I should have 5077RPM. Since it redlines at 7000, I can safely do 65 in 3rd. If I enter the highway in 3rd, and keep it there until I reach 65, I should have 5077RPM, sort of hard to measure, maybe a little more than 5000. If I notice that it is somewhere else, like 5300RPM or 5500RPM, then I know the effect drag has. If it reads 5500, then I should be doing 70.4MPH, but I'm only doing 65.

If you have a manual, then this is an easy test.... In a 4spd auto, my 3rd is like your 2nd, and most autos have OD, D, 1, thus, 4-3-1, no way to select 2. You could try it with OD and D, though. Or if you have a manumatic or SMG, well, thats a given.

If you actually try this, your peak efficiency (100% in a vaccum) will be when your calculated speed for the RPM your enging is turning divided by the actual speed is highest. I don't think it would ever actually be 1.00, though. (If I am going 65 and should be going 70.4, then the efficency would be 92.3%). Be interesting to see the difference in efficiency at 35 and 70. We all know city MPG is lower because of stop and go. This introduces idle, which gets 0MPG, and time in lower gears.

For anyone with a manual, I've done this before and it works fairly well, perhaps you have as well. If I'm going reasonably fast, atleast 35, works real well at 45, and theres a point that I'll have to stop at, maybe 1/4 mile away or a little less, I can shift into neutral or hold the clutch in, and since that lowers my RPM to 600RPM, and I'm still moving, the gas mileage for that little bit of roadway goes up really high. It's actually amazing how much kinetic energy is in a 2800lb vehicle moving 45mph. It can easily send you to that stop sign from 1/4 mile away, and you'll actually have to brake, its not a slow rolling stop. Anyways, done enough times, this can increase mileage (for example, done from 1/4 mi away four times, and if you drive around at 2500RPM average (maybe a little high, but I do get 24mpg driving with 2500RPM in 4th; sending it to 5th puts the RPM around 1800, and there isnt enough torque) but now are doing 600RPM and you got 24mpg average, now you should have 24 * 2500/600 MPG, or better known as, 100MPG. Remember, though, that 100MPG is only when moving from speed. Moreover, if you do that four times per trip, from 1/4 mile, on a trip that is 20 miles, the effect that 100MPG has on the average ends up being less so. If it were in a 10 mile trip, it would be more so. (E.G., 24mpg on 20 miles with this neutral thing for 1 mile, yields an actual MPG of (19*24+100)/20 = 27.8MPG, whilst on a 10 mile trip, (9*24+100)/10 = 31.6MPG.

Anyways, the only effect on mileage is drag once youre in top gear. If I swapped in a new final drive ratio of 3.33, my mileage would change accordingly (from 24 w/ a 4.06 to 29 w/ a 3.33), but I can't really do that. Driving slower should decrease the drag, by an amount that might or might not be significant (I'm taking physics in the fall and my high school physics teacher never got that far), thereby increasing the efficiency, thus making each gallon of gas go further. It's entirely possible that a car that is aerodynamically designed can get the same mileage at 100mph and 55mph. So, really, that graph is bull. Once you reach top gear, your mileage is at a general peak (if I can go fast enough to reach 5th, which is 35mph for me, I automatically get better mileage than the same speed in 4th). How fast do you have to go for drag to decrease it a noticeable amount? Depends on car design. I can tell you right now a boxy Astro van won't be able to go fast with the same mileage...maybe better luck with an Audi TT or Nissan Z, something a bit more aerodynamic.

What I find interesting in these discussions of 55 saves gas is the time factor.

While I believe you will get better gas mileage in most cars at 55 than at 70 mph, it will take you 27% more time to travel the same distance.

Even though your engine is burning less fuel per hour due to the slower speed; it is running for 27% more time which has to be taken in account.

[I am not considering what your personal time is worth for sake of this example.]

Let's say you drive 60 miles at 70 mph at 30 mpg. You have used exactly 2 gallons of gas.

Now you drive 60 miles at 55 mph getting 35 mpg. You have used 1.714 gallons of gas but it took 27% more time [70/55]

What is your fuel savings?