This is a followup to a thread I cannot find, in which I mentioned that the 55 mph speed limit had been preceded by 50. I cannot find that thread – Search seems to be a tad wonky.
A search in the NYTimes Index produced the following. Nixon started off by imposing a speed limit of 50 on government vehicles, requested state laws, requested voluntary citizen participation, and requested Congressional authority to implement a national limit. Many eastern states complied, starting with highways with special commissions (such as the NJ Turnpike) which could change the speed limit on their own authority, and trickling down as state legislatures acted. Most western states did not cooperate.
One issue raised was, indeed, that 18-wheeler gearboxes were such that 50 was less efficient than 55.
Eventually, the thing was passed by Congress, but the final figure arrived at was 55. The eastern states that had gone to 50 quickly moved up to 55.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams
No, but many states implemented a 50-mph limit in November and December of 1973, anticipating Nixon’s proposed 50-mph national speed limit, which Congress then changed to 55 when they actually passed the bill.
So I was correct about my original point, which is that the initial 50-mph limit was changed, largely due to the complaints of truckers that that speed was less fuel efficient than 55. (Indeed, one intermediate proposal was 50 mph for cars and 55 mph for trucks.) I was incorrect, however, in that the 50-mph limit, although partially implemented by the states, was never given force of federal law.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams
The argument you allege the trucking industry used does not hold water because of the gearboxes you mention. Multi-speed transmissions allow a truck’s engine to operate at the optimium rpm range for any given speed; the science of fluid dynamics tells us propelling a vehicle at a higher speed requires more energy by dint of the simple fact a greater volume of air must be pushed out of the way.
This was the whole idea behind lowering the speed limit in the first place, and I don’t believe Congress bought into anyone’s specious argument that a higher speed limit would increase fuel efficiency.
The 55 mph national speed limit was not a compromise between 50 and 55, it was a mostly unilateral and fiercly protested reduction from 65 (and in some places much higher)imposed by the Federal government.
I’m sorry, but now you’re just plain wrong. Are you seriously saying that gas mileage doesn’t change if you upshift or downshift?
18-wheelers have 12 forward gears; they don’t do that for a joke.
As to the early history of the national speed limit, read the N.Y.Times Index for 1973, under “Traffic” in Volume 2. Nixon asked for 50, and some states immediately implemented 50. But by the time it got through Congress, it had become 55.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams
Gas mileage does change, but it’s a function of the air resistance encountered, not the gearing of the powertrain. You seem to think there is some magical engine rpm/ gear combination that allows a truck to travel at 55 mph with better fuel economy than one traveling at 50 mph. You may be buying that proposition, but I think it’s a safe bet Congress didn’t. The same rules apply for a speed increase of 5 mph as they do for a 10 or 20 mph increase, and if the laws of physics somehow become inverted at 53 mph, I never heard about it.
It’s less efficient to to run an engine at 2,000 rpm in 12th gear than it is to run the same engine at 2,000 rpm in 11th gear because the vehicle is traveling faster and has to move more air out of the way, and more turbulent air at that. This is a well understood principle of fluid dynamics and I can never understand why people continually flout this basic fact when arguing to the contrary.
Reductio ad absurdum, this reasoning would have trucks tooling along at at 125 or 150 mph with the same fuel mileage if they only had a more powerful engine and a higher gear ratio.
What you’re not allowing for is that at a lower speed, it is necessary to downshift more often. And truckers claimed, and seem to have demonstrated, that, at least with with the gear ratios in common use in 1973, 50 mph resulted in frequent uphill downshifting that 55 mph did not need. Diesel engines have a very narrow range of efficient RPM’s. (Every engineer knows that reciprocating-piston internal-combustion engines are the work of the Devil to begin with.)
A brief summary of the actual history that can be gleaned from the N.Y.Times Index seems to be:
Nixon called for 50 mph.
Truckers complained that 50 mph was an inefficient speed, because it forced downshifting on hills.
The suggestion was made, and taken seriously, to make the limit 55 for trucks and 50 for cars.
Police and legal specialists complained that this would be unenforceable.
When the law finally passed, the limit was 55 for everybody.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams