[paulgato]

Originally Posted by ksa8907

How to you propose driving 500 miles in a day to go on vacation? 10 hours non-stop at 50mph or 6hrs 40min at 75mph. I will gladly pay $20 or however much it is to non be stuck in a car for an additional 3.3 hours.
Trust me, when the kids are screaming you just want to power that sucker up to 85 and set the cruise control.

Well, each to their own, but I decided long ago to start my holidays (vacations) the moment I set off from home, so although it may take me a bit longer to get to my 'destination', I don't lose out. I drive at a relaxed speed, usually avoiding motorways (freeways?) so I can enjoy the scenery. Avoiding motorways means we can stop and take a break at any time. Everyone is more relaxed, including kids, in my experience.

Yes, it can take longer, but not as much as you'd think. Often traffic congestion has a greater impact on the length of a journey than the maximum speed you decide to drive at. As I understand it (and I was told this by a guy who worked in traffic management in the Birmingham (UK) area) the optimum speed for maximum traffic throughput in a road IS about 50mph, as cars can safely drive much closer together than they can at higher speeds.

As for how much fuel you will save by "designing for a lower speed"... give some numbers, preferably with factual data. Otherwise its just meaningless words.

I apologise (not really) for not including footnotes or links to my sources. I think you only have to look at the prototypes of cars designed specifically for super-high fuel economy to realise they are all very low weight vehicles with super-small engines

As speed increases, the impact of air resistance becomes dominant, but at lower speed, weight is the overiding factor. Not sure of the physics, but I know for bicycles on a flat road, each doubling of speed increases the required power by a factor of about eight, and virtual all of that is down to wind resistance.

This from Wikipedia on air restance...

"Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power."

Internal combustion engines are most efficient at close to maximum power output, and least efficient at lowest power output, so an engine capable of overcoming wind resistance to push a car along at 120mph will have roughly eight times the power of an engine which only has to push the same car along at 60mph. At 50mph it will be running at roughly 10% load, whereas the smaller engine will be running at about 70% load. And that's for the same car, same weight, everything, including some ballast. to make up for the lighter engine.

And then at lower speed you can get away with far less weight in virtually every area of the car (chassis, drive train, suspension, brakes, body panels, everything) and since at lower speed weight is the dominant factor in fuel efficiency, you save fuel pretty much proportional to the weight you can save. (Half the weight, half the fuel consumption.) The drivetrain for example, from engine to tyres, only has to handle one-eighth the power if the design speed is halved, so all that stuff under the bonnet (hood) can in theory be one-eighth the weight.

Being forced to design for a 'normal' road speed of up to 100mph is a major constraint to designers wanting to maximise fuel economy. My grandmother's Morris Minor with 1000cc engine and little flp-out indicators with flashing orange lights on stalks like an insect was an aerodynamic disaster, had zero safety features, zero sound deadening (I still remember the whine of the gearbox) but it would get 50mpg easily, and that was in the 60's. Didn't go fast, but that didn't matter as there was much less traffic around and most cars were slow.

In no way do I want to be offensive, but perhaps in Europe where the countries and cities are much closer this is practical. I have to drive 120 miles one way each time my employer wants me to come to the HQ.

No offense taken. It's true, distances in Europe are often shorter, and fuel is also much more expensive - at least double usually.

...in the context of this discussion about safety, then I'm saying that if speed limits were lower, then no, we wouldn't need half that safety gear and we'd all still be a lot safer.