Centerfield

And as usual, she's wrong on the facts too. Every vehicle has a different sweet spot for fuel efficiency. Maximizing your fuel efficiency depends alot more on HOW you drive rather then how fast.

Next she'll be saying that we should only be producing size 9 shoes to get maximum industrial efficiency.

Shoes for industry! :-D (bonus points...)

My old '77 Ford van got its best mpg at 78 mph. 300 CID straight six. My '89 Suburban peaked at 67 mph. The 55 mph scheme was designed for the fleet on the road at the end of the Nixon admin. Not today. I was driving all over the country back then--it was the most widely ignored law we had.

As a non-fan of Hillary, is there any way I could get her to support bringing back Prohibition "wherever it can be required, and the people will accept it?"

While still boosting ethanol production? More likely it'd be a "don't drink, we need the booze to drive!" thing.....

She may not know that in order to slide by CAFE standards car fleets are manufactured and tuned to have optimal mileage at 65mph. That essplains Tully's suburban. As for the Ford van - aside from the high drag coefficient (what about .45 or higher?) and the fact that drag increases exponentially with speed (going from 65 to 78mph would be about 140% more drag) I can only chalk it up to one heck of a motor or more likely the fact that there were fewer beer stops. That's due to the inverse exponential need for beer as one drives faster. And we know how stop and go can kill mileage.

BTW those Nixon-era speed limits did help conserve millions of barrels of oil. However with the way everything is manufactured these days I doubt we'd see as much of a savings in fuel use if a similar limit were imposed today. Capping the limit to 70 would make sense from both a fuel savings perspective and safety (remember the 140%? it works for kinetic energy as well.)

I usually vote Dem. But that would lose me.

Actually it was due to the gearing and rear-end ratio and ignition setup and carb tuning, Marcus. Not that the 300 CID Ford isn't a great engine. It is--designed for hauling loads and vehicles much heavier than that van. Mediocre acceleration, but great torque and more efficient than a 351 V8. Also more dependable.

But that was a '77 vehicle. Real steel. NOT computer-controlled, like the 'burb. Drag is much less of a proportional factor when it's facing twice or more the rolling kinetic energy of a modern vehicle. As long as the fuel-optimal RPM/gearing range produces sufficient torque to offset the drag, you're golden. Drag hits modern vehicles much harder. Less weight, less rolling kinetic energy at speed. And that factory tuning you noted, controlled by that factory computer.

Beer stops? I had a cooler. :-)

We're going to get raided by MADD if this conversation keeps up....

As long as we are on the topic, my wife (w/daughter in car) got a ticket today for going 70 in the HOV lane. After her encounter with the officer, she set the cruise control at 60, was consistently passed by people in the regular lanes, and was tailgated by angry people in the HOV lane.

Pulling out a legal standard that I deal with frequently, I will say that enforcement of speed limits in this country is "arbitrary and capricious."

What do you expect from a politician who lives primarily in the neatly-compressed world of New England and the east coast? It's the same thing that gives me pause when they start talking about mass transit. The question of transportation policy reconfirms the wisdom of giving weight to states without reference to population in at least one part of government: it may be true, as they say, that fields o' corn don't vote, but a transportation policy that might seem pretty rational when everywhere you want to go is within an hour's drive starts to look pretty ridiculous when applied to a state the size of Montana (which, I seem to remember reading, is precisely why that speed limit was ignored in the west from the day it was instituted). This isn't a question that urban and esat-coasters can be allowed to resolve on their own.

Something else to note, although i'm not sure what its import is: Indiana recently (last couple of years) jumped its speed limits on the interstate from 65 up to 70. Before the change, I drove on the interstate at about 70; after the change, I drove on the interstate at...About 70. As you get into Indianapolis, the speed limit inexplicably drops to 55 on an eight land highway, and stays that way once it drops back down to a six-laner heading into the city. This is widely-recognized as being patently absurd, with the net result that everyone - including the police - drives that streth of road at...(you guessed it) about 70. It is ludicrous that the state forces drivers to choose between driving safely or driving at the speed limit; speed limits should be set based purely on the practical speed that the road suggests, and when they are so set, it seems to me that people do not speed up accordingly.

hey I know a lot about rolling K. You should try the San Mateo Bridge's cross winds on a 83 Honda CB1100F vs a 90 Suzuki DR350S thumper.
hoooyaaaaahhhh. But like you said - gearing and tuning (pre computer) is what did it in the olden days.

I had a 70 Fairlane with a straight 6. It was a midwestern rustbucket that emigrated West. If it ever got hit it would have been been a poof of dust and rust. Best speed was in the upper 60's. Got better mileage (18mpg) when you had the heat on. Good old Carnot. And you could literally put a bunk bed in the engine compartment next to the motor. OR put in a second motor. Real easy to work on. Big wide car. Great for drive-in action. I still have a Ford. My current is a 90 Escort wagon. Lots of steel but gets over 32 on the freeway, even with 165K on the motor. Come to think of it, I wonder if my girlfriend is up for a trip to the drive-in?

oh, the beer stops I was thinking of, weren't the ones that could be solved by a cooler. Unless you used it to ....ummmmmm......

55 will not enamor any Arizonans to Hillary, Democratic or Republican. Folks want to get from Phoenix to Flagstaff or Phoenix to Tucson in UNDER 2 hours.

Hell;we just had a big controvery with Scottsdale installing photo-radar on one of our expressways. Theoretically they would flash you if you exceeded the 65 MPH limit but they admitted they won't "count" unless your over 75. Even that pissed off a whole lot of folks.

In my driving down in Arizona and the gorgeous scenery that is abundant along many a highway (my late grandmother used to work at Verde Valley school down in Sedona) why be in such a hurry?.
Yeah, I know, everyone is in a hurry or a commute. The simple act of driving, even on a vacation becomes a commute. Then it's no longer a vacation but a get there, mechanically enjoy the destination and get back.
I did an experiment in 86. I biked up the West coast on my CB to the World's Fair in Vancouver. When I got into Canada I decided to just go speed limit as my max speed. To let people go by and relax on the ride. Well I got even greater mileage (from 45 mpg to 50 on a 5 gal tank) but even more entertaining was looking at the disappointed faces of the many RCMP that hid in bushes, median strips and everywhere else with radar guns pointed at me.
And the travelling became a nice vacation as it should have been. Also kept me from being whacked by deer in Idaho - I decided not to pass a logging rig near sunset and literally the next minute 2 deer decided to cross the road. One made it.

Ever since then, even on the Sunday Morning Ride that I did for years, I always took time to take it easy, take it all in.

I think it was Robt Lous Stevenson that aid something about it's not the destination but the journey.

Hee hee. Pit stops were no prob. Pull to the side, open both side doors, and stand between 'em. Yeah, side winds are always worse than headwinds or tailwinds. Especially in a van!

I think the best engineering improvement in modern cars is those drop-down rear seats that give most cars actual cargo capacity, and make those little wagons mini-campers in a pinch.

Drag is much less of a proportional factor when it's facing twice or more the rolling kinetic energy of a modern vehicle. As long as the fuel-optimal RPM/gearing range produces sufficient torque to offset the drag, you're golden. Drag hits modern vehicles much harder. Less weight, less rolling kinetic energy at speed.

I hate (love?)to get off on a tangent like this, but I need a bit more detailed explanation of the above, Tully. Are you saying that only the percentage of the total replacement energy due to drag required from the fuel is greater in lighter vehicles at a given higher speed, or that the absolute replacement energy required due to drag is greater (assuming the same vehicle size and form)? If the absolute drag force, and therefore replacement energy, is the same, which I think it is, what accounts for the greater percentage of replacement energy due to drag in a lighter vehicle? IOW, what other absolute replacement energy is (or energies are) less in a lighter vehicle at a given constant higher speed in that case?

Dunno about all that, WHQ, certainly not before my second cup.

I'm saying that given equal air drag, it requires less change-of-throttle to keep a 3-ton Cadillac rolling at speed than a Kia Rio, and that the biggest spoiler to optimal fuel efficiency is change-of-throttle. That it is easier to maintain a constant optimum speed in a heavier vehicle against a given amount of air drag, resulting in less fuel-gobbling variation. If both are going 70 into a headwind on the flat, if you put them in nuetral the VW will slow down quicker. Air drag depends on surface profile, kinetic energy on mass.

I'm NOT saying that the Kia gets worse mileage at speed. Obviously. But if you're considering only air drag, the heavier vehicle suffers less change-of-speed from the same amount of air drag, and requires less RPM variation to maintain constant speed against that drag, and can spend more time in the optimal fuel-efficiency constant RPM range.

The heavier vehicle will still require more fuel all around, assuming equal horsepower, torque, and gearing. But it will suffer less drag-related fuel-robbing throttle variation.

I guess that's intuitive enough for me. If it's simply a matter of how transient the throttling is, I can see that.

I probably had a more ideal model in my head, assuming constant speed and trottling against a given force. Work = Force x Distance. Same force, same distance, same work.

But thinking about it again this morning, force also equals rate of change in momentum. Thus the greater slowing of the lighter vehicle against drag. The change in momentum for two vehicles of differing mass would be the same, let's say, for illustrative purposes, half the mass times twice the velocity. But the kinetic energy, being in proportion to the mass and the square of velocity, will change, effectively, in inverse proportion to the mass. So the lighter vehicle will tend to lose proportionally more kinetic energy, half the mass times twice the velocity times twice the velocity.

So even my ideal model without any actual changes in velocity has it's problems (for me, anyway). The velocity changes described above can be taken to the infinitesimal (dv/dt) and be applied to a constant-speed vehicle. But I still have to resolve what appear at this point to me to be contradictory analyses, the force-times-distance-equals-work approach and the force-equals-rate-of-change-in-momentum-applied-to-kinetic-energy approach.

But what are your ODDS of picking a low-drag high-mileage vehicle... 1 out of 2 or 1 out of 3? ;-)

The problem with applying either the "force-times-distance-equals-work approach" or the "force-equals-rate-of-change-in-momentum-applied-to-kinetic-energy" approach is that they both assume that the engine is perfectly efficient -- or, at least, equally efficient at all speeds, in translating the energy of the fuel into force. Which, in the real world, it ain't. This is why mechanical engineers get paid: to figure out how to make engines more efficient. The good news, career-wise, is that there is still lots of room for improvement! ;-)

I don't think so, wj. It assumes a given speed, drag coefficient and, for lack of a better phrase "frontal area" (i.e size and shape). Those things will determine the drag force. The engines efficiency is whatever it is at that speed. I'm simply looking at the effect drag has on two cars of different mass. It's a minor tangential point, as Pat alludes to in his humorous comment.

Yep. The practical result of which is that Big Steel is slowed down less by a given amount of drag than light fiberglass, all else being equal. But of course Big Steel still eats more gas anyway, it just does it with less throttle & fuel use variation due to drag.

I don't know about the scientific rationale, but it seems to me if you set the speed limit at 55, people will drive 65. If you set it at 70, people will drive 80. Of course, there are always people that will drive as fast as they can regardless. But even the oil companies say that the way to bring gasoline prices down is to drive slower.

Click and Clack say that you get better gas mileage at lower speeds. But, then, they are a couple of left-wing ideologues anyway. -:)

Marc,

Again, it all depends upon what the "sweet spot" is for your particular car. Driving at that speed will make the most efficient use of fuel. However, it matters even more HOW you drive then how fast. Every time you step on the brake or jam down hard on the accelerater you waste fuel. The best use of fuel is to drive at a constant speed.... with slow, gradual decelration and acceleration when neccesary. This will save you alot more fuel then even trying to stay in your cars "sweet spot".