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Thus during WWII this was the Speed Limit adopted. By the time of the 1970s most people were driving Automatic Transmissions which would NOT get into their highest gear till after 35mph (and at about 35-40mph switch between third and Second gears). Thus 55 was adopted to get around this problem.
I have traveled in a Five Speed Standard at 25 mph in Fifth, and my vehicle would NOT buckle until I came to any form of grade, at which point I have to downshift. The key is getting your Vehicle into its slowest speed while in the transmission's highest gear and keeping it in that gear even if you get some buckling. I notice when I stay at about 45mph I get about 25mpg, but if I go faster in my Jeep Wrangler the fuel economy goes down hill (I live in the Country and can put my Jeep in fifth gear and keep it in Fifth for 20-30 miles at a time, very few stops etc). In the early 1980s I once did 25 mpg in a three speed standard Chevrolet 3/4 ton pickup (I was going 25mph on a long stretch of flat Interstate just to see what fuel economy I would get, normally that truck get 12 mpg).
My point is you will get better fuel economy at the lowest speed when the Vehicle is in is highest gear. This is harder to do with Automatics since most will shift down WAY before you would have to if the transmission was Manual. When my sister was working in Detroit in the 1990s she was told by people in the Auto industry that the industry was using the new electronics in the newer Automatic Transmission to EXTEND the time the transmission would stay in second gear. This was to make the transition from first to overdrive smoother. The Down side was it wore out the transmission quicker, the upside it permitted such transmission to get higher EPA mpg ratings even as real mpg rating suffered do to the longer time in Second gear.
Today if you look at the EPA mpg estimates, the automatic transmission is rarely less then the standard transmission in the same vehicle. In real life this is NOT the case, and it is worse if you compare drivers of the Automatic and Standard Transmission if both drivers are trying to maximize mpg (And Hybrids do better on the test then they do in real life driving, through some drivers of Hybrids who try to maximize fuel economy do match the EPA tests).
As to the Five and Six speed Automatics, while they get better fuel economy at higher speeds than the old 3-4 speed automatics, to operate efficiently they MUST get to the speed where the vehicle stays in the highest gear. If that is 50 mph there must get up to that speed to get that fuel economy (and the EPA test occurs at 50 mph, thus auto makers tend to design their automatic transmission for max fuel efficiency at that speed).
People have been bragging about the new 5-6 speed automatics for years saying they get the same fuel efficiency than standard transmissions. The US Army even bought them for its trucks (But seems more to do with training time then fuel economy, even when I was basic the job of Truck driving went to people who ALREADY KNEW HOW TO DRIVE A STANDARD. The Army did NOT want to waste time training people to drive standard transmissions. Now the Army had a problem getting enough people who knew how to drive a Standard when I was in the Army in the early 1980s, it is worse now and for that reason, more then fuel economy, the Army has switched to Automatics in their trucks.
While the Army has converted to Automatics in their trucks, most civilian truckers still opt for standards, and the reason is greater fuel economy. The fuel economy difference still exists AND INCREASES IF YOU KEEP THE TRUCK AT ITS SLOWEST SPEED WHILE IN ITS HIGHEST GEAR. Now most truckers do NOT operate their trucks at the slowest speed in the truck's highest gear (Truckers make money by getting the shipment to the customers at the quickest speed, the price of oil is NOT so high that the truckers are looking at ways to get better fuel economy at the present time), but when the trucks are operated that way you get much better fuel economy.
My point here is given that the only fuel estimates people may use in the US, are the ones provided by the EPA, and since those tests since the 1970s have been geared to vehicles going at 50 mph, there is NOT a Small Car or Truck that does NOT get its best fuel economy at 50 mph or less (With the major exception being high end Sports car that go over 200 mph, but such cars are RARELY purchased by most Americans). No auto maker will make a car for general sale in the US that does NOT do its best fuel usage at 50 mph or less (The EPA test use slow acceleration and de-acceleration do to testing limitations of the test equipment first adopted in the 1970s, modern equipment can test at more normal speeds, but the equipment available in the 1970s could not, and the test adopted were set up by Congress based on what the EPA could do in the 1970s).
Given the slow acceleration and slow De-accelerations of the EPA tests, any car meant for the general public, will gets its best fuel economy between 35-55 mph (I suspect that Daimler's "Smart Car" only received a 47 mpg rating under the EPA tests, do to the need to over-rev the engines to get it up to 50 mph, the SMART car was design for much slower speeds than American cars). Since most people do NOT do gradual acceleration and gradual De-accelerations AND go over 50 mph, they fuel economy will be much less than the EPA test results. This also mean that I do NOT see any car, aimed at the General population, getting better fuel economy at speeds over 50 mph. If you believe you are getting better fuel efficiency at higher speeds I would check the calculation, something is wrong, for GM and the other auto makers are NOT going to put themselves at a disadvantage when it comes to getting the highest EPA mileage they can, and that is achieved at 50 mph.
Personally I suspect you are operating the Trans Am at high speeds (It is a Sports car), and as such NOT keeping it around 50 mph except for brief periods when the engine is accelerating or De-accelerating. Thus when you operate the Car at 50 mph your car is NOT going at a steady pace at that speed, by speeding up to a higher speed. Acceleration ALWAYS require more power than keeping a car at a set pace (And in a Trans Am, 50 mph feels like you are standing still). I suspect when the Trans Am was tested, it was tested like any other car by the EPA with slow acceleration and slow de-acceleration. Something no one driving an Trans Am does (and most drivers do NOT do no matter what car they are driving). Given the above if you want maximum Fuel Economy you should for the engine/Transmission combination as it was probably designed by GM for its best fuel economy, i.e. at 50 mph for at that speed, and only at that speed are you operating the car in the matter that reflects the EPA tests WHICH ALL CARS MUST GO THROUGH.
In simple term a speed limit, if obeyed by most people, will save a good bit of oil, but it will require the price of oil to be much higher than it is now, especially given the hostilely in this thread to the mean idea of reducing the speed limit.
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