I guess if he scores one, he'll just radio ahead to the rest of the trap.
Posted on 05/01/2005 6:19:00 AM PDT by MississippiMasterpiece
I discovered the "sweet spot" in my 1997 Saturn SL1 while on a trip up north. (I won't say what provinces).
Did a hundred for one complete gas tank... got 45+MPG!
At 75 MPH down here I get 39 to 40 MPG.
At 65 MPH I get 41 to 42 MPG.
I don't know what it gets at 55 MPH.
The thing has 415,000+ miles on it now, still going strong without a squeek or a rattle.
But, I think if fuel prices remain relatively high, Americans will be willing to look at diesels in automobiles again, once they are made aware of the current state of diesel engines.
After all, they've been increasingly popular in light trucks in this country.
"Mr. Hwang" can kiss my grits. This is about as useful as recycling gum wrappers. Gibbering idiots.
Pumping up the tires can be a big deal as most recommended settings are based on giving a good ride and not being able to generate enough cornering force to roll the vehicle over. If your tires are below the recommended settings, they are a long way from being optimal for fuel economy. It may only be a few percent effect, but it is available for FREE. Be careful though. (;-0>
There could also be some other variables. As the Federal emissions and fuel economy tests are performed at speeds below 65, it is possible to run slightly lean at higher speeds and gain a few percentage points of engine efficiency. Note that it is difficult to fill the gas tank to the same level every fillup. A half gallon fill difference (1/4 gallon both ways) can make a big difference in the calculated fuel economy. Running tests can result in lots of variability unless the measurement system is quite capable and lots of variables are well controlled. That's one reason why they say "your mileage may vary".
The power absorbed by rolling resistance will be proportional to velocity, but the force it produces IS (roughly speaking) constant as a function of speed. Similarly, the power consumed by by aero drag is proportional to the cube of the vehicle's velocity, but the retarding force is proportional to the square of the velocity.
This 55 mph speed limit is this East Coast mentality of "we know what is best for you" and you will like this "one size fits all". Only the Easterners are so arrogant !
"Double your speed and you quadruple air resistance."
I don't plan on running 110 MPH.
Higher taxes would just make it more devastating for those of us who already can't afford gas to go to work.
I guess if he scores one, he'll just radio ahead to the rest of the trap.
For a normal vehicle that might be true if the difference in the speeds was say 60 vs. 55 but for 55 and 75 the relative velocities for a given Cd preclude that.
OK, at 75 mph then. 75 is 36% greater than 55, but the air resistance goes up to 1.362=1.86, and increase of 86 percent. It's the same principle at any speed over 55.
Another reason to despise the French, and anybody that would advocate more TAXES to encourage (FORCE) that myth called conservation.
The NY Times should save energy by shutting off their printing press.
It depends on the car. If the car has poor aerodynamics, the aero drag will overwhelmingly dominate the calculation. If the car is slippery to the air, the rolling resistance will play a larger % role until the speeds get sufficiently high for the aero drag to dominate.
But for illustrative purposes, lets assume at 55 rolling resistance and aero drag are roughly equal to each other. (That may not be exactly accurate, but it varies from car to car, and provides us with a convenient basis for our calculation.)
A 10 mph increase from 55 to 65 increases aero drag by about 40%, but since we assumed that at 55, aero drag = rolling drag = 50% of total drag, then the the total drag increases 20%.
If we go from 55 to 70, aero drag increases 61%, so total drag goes up half that: 30.5%
If we go from 55 to 75, the aero drag goes up by 85%, or total drag increases by 42.5% compared to 55....
And since fuel consumption at constant speed (assuming no hills) is proportional to the total drag, you get some idea of what happens to fuel economy as speed increases.
Caveat: the increase in drag and hence fuel consumption can be offset to some extent by design choices that will optimize fuel economy at higher engine speeds versus lower engine speeds. From a volumetric efficiency stand point, and engine produces maximum energy out per unit of the energy of the fuel used when it runs at wide open throttle at the engines torque peak. Depending on the engine/valve timing and gearing in the drive train, it is possible to have a car's engine be more fuel efficient per unit of fuel consumed at a higher speed, but this is quickly offset by the exponential rate of growth in the aero drag. Also, given the government mandated mileage tests for all models of cars, and the way they are calculated, it would make no sense for a auto manufacturer to design a vehicle with an engine/drivetrain optimised for high speed driving, because it likely would get lower ratings on the Federally mandated mileage tests than if they design it for a compromise between "City" and "highway" driving.
That said, fuel consumption just isn't an appropriate standard by which to set speed limits; safety arguably is, and if we accept safety as the prime issue in decideing speed limits, 55 is a killer on superhighways -- it's just too slow.
How does the profile figure into that calculation?
Aero drag is proportional to frontal area; if you reduce frontal area by 10% and keep the same drag coefficient, the aero drag drops by 10% (now you know why some NASCAR team once built a 9/10ths scale car, and reputedly beat the crap out of their competitors; that was one of the reasons they introduced the use of body templates, to put a stop to aerodynamic cheating).
Similarly, if your car is huge in frontal area, the effects of aero drag will begin to dominate at a slower speed than for a car with smaller frontal area.
Caveat: the above assumes vehicles with similar coefficients of drag. That's why a motorcycle, which weighs much less and has much less frontal area than a car, gets mileage that's very similar to that of a car. From the standpoint of a bunch of air molecules, a motorcycle and rider represent something like a brick trying to punch through the air, whereas modern cars have MUCH lower drag coeffecients than motorcycles. They may be bigger, and thus affect more air molecules than the motorcycle, but they abuse the air molecules much less.
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