Bringing back the 55 mph limit?? Read this!

[Greg Amy]

...my own experience shows a saving in consumption...Therefore my observation is that lower speeds = higher mpg.

I agree: driving slower uses less fuel (with some notable exceptions, such as higher-performance vehicles geared for higher speeds). However, the issue is not whether driving 55 saves fuel versus 65 - we can agree that it does - the issue is whether a 55 mile per hour speed limit saves fuel versus a 65 mph speed limit.

The answer is: it doesn't.

Why not? Because people don't drive at artificially-low-set speed limits, especially 55 (or even 60 or 65) mph on an interstate-quality highway.

Take a look at this chart:
http://www.eia.doe.gov/emeu/aer/txt/ptb0208.html

Note the "miles per gallon" columns. Note that it gradually dropped from 1949 to the late 70's, when CAFE was initially implemented (initially 1975, phased up standards to 1990). From there it gradually increased to 1990-91 when the last levels of CAFE were implemented. Notice ZERO change (actually, an increase in fuel economy) when the NHMSL was repealed in 1995.

While driving 55 mph will save fuel when compared to driving 65 mph, the 55 miles per hour speed limit had ZERO effect on fuel consumption simply because people ignored it wholesale. And, no amount of enforcement binges, tax rebates to the states, and mass-market media campaigns to the contrary changed that.

The majority of people will drive at a speed that is safe for the conditions; on interstate-quality highways that speed is well above 55 mph. Mandating the speed limit to 55 mph will have negligible affect on the speed people drive, proven by the "success" of that speed limit during the Great Experiment. Since people are not driving at 55 mph, there will be negligible affect on actual fuel consumption. Since the stated reason for dropping the speed limit to 55 is to slow people down to save fuel, and it has been proven time and again to not work, setting the speed limit at this artificially-low speed will have no affect on the amount of fuel used.

Ergo, why make criminals out of people with no positive effect(s)? You would have just as much chance getting people to drive 55 now within the existing speed limits (which is perfectly legal to do) as you would trying to get people to drive 55 with a 55 mph speed limit.

In other words: zero.

Greg

[LD71]

Greg,
I appreciate your point, it would appear that voluntary efforts to save fuel would have a better result.

The chart has some interesting data. It would appear that "trucks" would be lower-hanging fruit---do you know what vehicles are included there? I assume diesels are part of the mix....surprising to me that mpg would remain at such low levels...

LD71

[Greg Amy]

It would appear that "trucks" would be lower-hanging fruit---do you know what vehicles are included there? I assume diesels are part of the mix....surprising to me that mpg would remain at such low levels...

Larry, those are the local/OTR semis and large container/moving trucks, like local delivery. The most likely reason we don't see improvements there is simply physics: these trucks move large, aerodynamically-inefficient loads, and that takes a certain amount of power.

We've seen some aero improvements in the last few decades on OTR trucks, but nothing revolutionary; the simple reason for that is our infrastructure and laws/economics don't allow it. One of the biggest revolutions in shiping is the containerized transport, which, for the sake of packaging, are large rectangular, square-edged boxes. While these may fit into a ship quite efficiently, and allow pre-packaging and easy transport, they don't make for very aero loads on the highway behind a semi. Further, we have laws in place that limit lengths and such, therefore limiting what kind of attachments we can economically add to trailers to make them more efficient. Finally, our infrastructure is set up to load/unload/handle these square boxes, so - for example - adding large aero attachments to trailers becomes an impediment to efficient transport (plus they are expensive and are easily damaged when a driver isn't paying attention).

We've seen some aero improvements in the trucks themselves over the years; compare a 1970 Peterbilt to, say, a 2008 Mack and you'll note a major difference, in terms of frontal blending and cab-to-trailer flow control. But, in the end, the big box in the back is X-by-Y-by-Z size, and nothing short of a total shipping/transportation overhaul will improve that.

Finally, while I'm no diesel mechanic, it does not appear to the untrained eye that any radical or revolutionary changes have happened to the mechanics of the truck fleet. They're still large frame trucks with basic diesel engines, that need XXX horsepower to pull YYYY poundage. Short of the obvious changes of direct injection and computer control (I'm assuming they're using that now) there's not a whole lot you can do to a diesel engine to improve its BSFC.

Probably the only thing the trucking industry can do to improve its efficiency is economies of scale, in terms of total load per semi (more weight, more trailers.) But, that's limited by highway capability and load limits. And, as we've seen in the past, any time load limits are raised we don't see improvements in efficiency, we see increased total load carried (i.e., allowing tandem trailer didn't cut the number of trucks in half, it doubled the amount of load carried by the existing truck fleet).

The ultimate in economies of scale would come if we, say, hitched more trucks together, maybe letting one driver in front carry 3 or more trailers. Then, to improve safety we could give them their own roads, discrete from cars. To keep them from having to change speeds all the time (their most-inefficient mode) we could give that road right-of-way over others. And, maybe, we could embed some kind of directional control (either computer-assisted or maybe even something as simple as a mechanical "rail" of some kind that the truck would attach to and follow) to keep the on their road. Finally, since hybrids are all the rage these days, why not make the TRUCKS hybrid? Maybe go diesel-electric, where the diesel generates the electricity, which drives the wheels? And, since we now have right-of-way, directional control, and constant speeds, maybe we could hitch up a few more trailers to be even more efficient; if we built a big enough engined "truck", think we could get as many as 25 or more trailers?

Maybe even a "train" of 50 trailers or more, efficiently pulling the whole load, controlled by one person (with a backup on board so they wouldn't have to stop), and done with a highly-efficient diesel-electric hybrid engine?

Yeah, I know: wishful thinking.... - GA