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The only way that could happen is if you have a "continuously variable transmission". Do you?
GL2
Interpreted his remark as indicating that the rpm is essentially the same at or near both speeds, and not that there is a mechanism that assures that it is at exactly same rpms at any speed. Which is quite possible with 6 or 8 ratios.
But if Why_ is running at the same rpm's, thats revolutions per minute, which is fuel consumed per minute. On a 100 mile trip, his engine is running at constant speed for 1.54 hours if he goes 65, but 1.82 hours at 55 mph. If his fuel is turning the engine at the same rpms, the engine will use the same fuel per hour, but run 0.28 hours longer, using that much additional fuel.
In this case, yes, I can see the point. It's pretty rare that a vehicle would not be turning more RPM at 65 than at 55, not many vehicles that I've driven would have a shift point that high up. Maybe these new 6 or 7 speed automatics do. I've never driven one. But then again, most cars I drive tend to be Y2K or older.
Interpreted his remark as indicating that the rpm is essentially the same at or near both speeds, and not that there is a mechanism that assures that it is at exactly same rpms at any speed. Which is quite possible with 6 or 8 ratios.
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Yes with more gears
the RPM spread from one gear to the other is closer but there will still be a difference in RPMs at different speeds while in the same gear. A "continuously variable transmission" keeps the RPMs of the motor constant while changing the gear ratios. There are several cars with the CV Transmissions made. However as far as I know none are used with a V-8 motor.
GL2
Whatever the particulars are, there seems no doubt that speeds above 55 can be handled today with a great deal more safety and economy than in the '70s when the 55 was thought useful. Even things like tire tread technology and intense lighting make speeds well over 55 in adverse driving conditioins much safer than 30 years ago.
Nevertheless, I would set 70 as an upper limit, with no tolerance. I can see no reason why anyone would ever need to go faster than that, reaction times are still the same as ever, and there are a lot of cars and drivers on the road that can't keep up with a 75 or 80 mphh traffic flow. The most important factor in highway safety is to have as many cars as possible all going at the same speed, and that has to be a speed that is comfortably attainable by all cars and drivers. Going 75 instead of 70 would knock maybe 30 minutes off a drive from Chicago to Atlanta, and how critical can that be?
I bought a 1972 BMW 2002 in Jan 1972 ... shortly before the 55 speed limit went into effect.
In our Western states, cruising the car at 65+ mph, I got 28-30 mpg.
Driving the same roads under similar weather conditions at 55 mph, the best economy I got was 25 mpg.
Why the drop? because the intake/exhaust system, compression ratio, cam tuning, and ignition advance curves were optimized for 72 mph cruising in 4th gear. The volumetric efficiency of the motor was better at closer to 4,000 RPM than the lower RPM at 55 mph in 4th.
You could really tell the difference in the throttle foot pressure driving at the faster speed, and the car was able to go up and down rolling hills (such as I-25 North of Denver) without much change in throttle pressure to maintain the faster road speed. At 55 mph, the car required a substantial increase in throttle pressure to maintain 55 mph going up the hills, and that greatly affected ... adversely ... the fuel economy.
I tried changing the car over to the two-barrel carb/intake manifold of the later 2002's to get some of the cruise economy back, but it wasn't effective ... and I didn't like the power transition of the vacuum operated secondary opening compared to the original one-barrel carb.
Similarly, I've compared fuel economy figures on my MB's ... diesel and gas models, 4,5,6 cylinder cars ... and all turn in comparable or better fuel economy at 70+ mph as they do at 55 mph. I even did a trip (Cheyenne-SLC, which I do a lot of times) on my cruise control at 55 mph in my 1982 MB 300Dt and got comparable fuel efficiency as I do cruising the car at 75 mph. If I'd have saved a couple of gallons of fuel to do the trip, I might have considered a lower cruise speed for the open road and a day-long trip ... but I didn't. I've also compared fuel economy on several of my motorcycles, and have come up with essentially the same result ... my 1969 MG Ambassador turns in 55-58 mpg cruising at 55, 65, or 75 mph.
OTOH, my diesel powered pick-up trucks get significantly better fuel economy at 55-62 mph compared to 75 mph cruising. I attribute that to the designed efficiency at 2,000 RPM operation compared to much higher rpm's at 75 mph ... and the aerodynamics of a brick outhouse, especially when pulling a loaded trailer. That's why my trucks are used strictly as trucks and not as transportation vehicles.
OP, there's way too many design and efficiency factors at work in modern designed cars to simply equate higher road speed (at the comparison of 55 mph to 75 mph) to more fuel consumption/fewer miles per gallon ... and it's been that way in my personal experience since the early 1960's on a number of vehicles manufactured from USA, Brit, France, Germany, and Italy ....
One would also have to look at the developments of safety features in modern cars to recognize that they're a lot safer post crash than cars of the 1960's-1970's, which made the fleet when the fed administration lowered the speed limits to 55 mph. Just about anybody's car of the last couple of decades is infinitely more survivable at 75 mph than the best cars of the early 1970's ... and the more recent cars with side air bags and better structure are yet safer post crash ... and better pre-crash in accident avoidance capability (ABS brakes, suspension controls, traction controls ....).
I would rather see efficiency of design and gearing than in speed regulations.
Air resistance (AR) = fA x Cd x 0.00256 x speed squared (in mph). So yes, speed increases air resistance, but frontal area (fA) and drag coefficient (Cd) both play into it, and those are things that CAN be controlled by good design.
And you can require the engine to work under less load (less fuel consumption by reducing power need:
Power (rwhp to overcome AR) = fA x Cd x .00256 x mph cubed / 375
While it can be difficult to improve frontal area due to engine bay width and wheel track, you could REALLY improve Cd. A Honda Insight is .25 versus a Hummer at .57. Takes a LOT more power to push the Hummer through the air. However, a Hummer (or other SUV) with the SHAPE of a Honda Insight would be much easier to push through the air.
So I would much rather see funky shaped vehicles with 10 speed transmissions than big square shapes with 4 speed transmissions slowing to 55 to save fuel economy...
While it can be difficult to improve frontal area due to engine bay width and wheel track, you could REALLY improve Cd.
A lot of good points (and good math) you bring up there. About that frontal area... do you think maybe Volkswagen had the right idea in the olden days about putting the engine in the back of the car? That could potentially allow for some drastic changes in front end design, could it not?
Whatever the particulars are, there seems no doubt that speeds above 55 can be handled today with a great deal more safety and economy than in the '70s when the 55 was thought useful. Even things like tire tread technology and intense lighting make speeds well over 55 in adverse driving conditioins much safer than 30 years ago.
Nevertheless, I would set 70 as an upper limit, with no tolerance. I can see no reason why anyone would ever need to go faster than that, reaction times are still the same as ever, and there are a lot of cars and drivers on the road that can't keep up with a 75 or 80 mphh traffic flow. The most important factor in highway safety is to have as many cars as possible all going at the same speed, and that has to be a speed that is comfortably attainable by all cars and drivers. Going 75 instead of 70 would knock maybe 30 minutes off a drive from Chicago to Atlanta, and how critical can that be?
It's not a matter of speed, but a matter of distance. How much distance are you from the car in front of you, how far from your rear bumper is the car behind you? Even at 65 mph, if you're running nice and tight together, and the first person in line stops for some reason, you end up with a pileup.
Speed limits in this country have remained relatively stable for the last 20 years. Automobile technology hasn't set still all this time. Cars have gotten safer, more fuel efficient, and capable of sustaining higher speeds in a variety of conditions.
If anything, speed limits should go up. I've asked a few people why speed limits stay the same. Generally, its because its the minimal safe speed for road conditions that normally occur. So if its raining, its safe to do 70 on the interstate. If its not raining, it is safe to do 80 to 85.
Personally, I feel that we should have changing speed limits. With traffic cameras, and solar panels, it would be easy to set up speed limit signs that change with the road conditions. So, if a controller or a computer sees that you are on a highway with little traffic, and its completely dry, and perfect driving weather, your speed limit might be 100.
The main problem with American driving, is that we want to drive all over the road. The autobahn allows you to drive at faster speeds, but more restrictive movements. You don't ever pass on the right there, and you always use a blinker when changing lanes. You also don't ride in the left lanes either.
You'll get a ticket quicker there for those things, where here in the states, we sacrifice speed for the ability to drive all over the road.
The Autobahn, to me, is a very bad idea. What it is, thousands of miles of third-lanes, which serve nobody except the guys going 110 mph in Murcielagos and Carreras. Nobody else is allowed in that lane. I made the mistake of going into the third lane once to get around somebody in the second lane. I checked to make sure there was nobody coming, and by the time I got around him, there was a bank of eight flashing Bosch highbeams expanding in my rear view mirror. It is not without its mishaps. Every hour of so, the scenery features scattered BMW parts strewn for a hundred meters past a hole ripped in the chain link barrier fence where somebody lost control.
There will always be drivers in the US in cars that are incapable of speeds over 65, or driven by drivers unwilling to drive that fast, and a 30 mph differential between two cars on the same road is like speeding through a parking lot at 30 where drivers are backing out of parking spaces. Most US interstates are still two lanes each way. It would cost too many billions of dollars to widen them all to accommodate a traffic lane going over 80 and one going under 70 and most going in between. In the past 20 years, I don't think I've ever owned a car that was capable of sustained speeds of over 75, and I was unwilling to drive them that fast.
If you increase your speed from 60 to 90, you have cut your reaction time by one third. Your car goes half again farther before you can react. Which is the same as driving 60, but so drunk that your reaction time is reduced by a third. Is that what we want? In my experience, driving 100 miles at 75 is more exhausting than driving 100 miles at 65, because of the increased demand on my attention and response, even though I am behind the wheel a shorter time.
Where is it that you need to go at 85 or 90 mph, and how much sooner do you need to get there?
Last edited by jtur88; 12-31-2009 at 05:54 PM..
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