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Old 04-09-2008, 12:10 AM   #29
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Quote:
Originally Posted by overlander63
Way back when, 50 was considered optimum for cars, and 60 was considered optimum for large trucks, as they were supposed to be geared to operate more efficiently at 60. President Nixon ordered the 55 mph national speed limit as a compromise between the two. Of course, he also said he wasn't a crook...
I do know that current EPA MPG highway test speed is 48 mph.
let me try and derail this thread into a political quagmire . . . (that was a joke).

i'm remembering that it was carter who changed the speed limit to 55 during the "energy crisis" (?). am i remembering incorrectly?
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Old 04-09-2008, 01:10 AM   #30
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As an emergency response to the 1973 oil crisis, the U.S. Congress and President Richard Nixon imposed a nationwide 55 mph (90 km/h) speed limit in 1974 by requiring the limit as a condition of each state receiving highway funds, a use of the Commerce Clause of the United States Constitution[3].
Wikipedia speaks.
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Old 04-09-2008, 07:20 AM   #31
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Quote:
Originally Posted by Gen Disarray
I just saw in my travel notes that while towing 4th gear 1,800 RPM was 45 MPH and in 5th gear 1,900 RPM was 65 MPH. I suspect that 60 MPH would put pretty close to the 1,800 RPM sweet spot (if thats where the sweet spot is).

Is maximum torgue what I am looking for?
Rodney, if you look at the curve I sent you, you will see that peak efficiency is just below peak torque on the RPM scale.
Intuitively, you want to think they are one and the same...but another way of looking at it; peak power would require more fuel...
Interesting that your experience (trip log) pretty much agrees with the other oil burners...

Hope this helps.

Bill
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Old 04-09-2008, 08:03 AM   #32
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I think a more significant factor in mpgs is terrain and wind. I do, however, try to tow at 55 to 60.

I bought a K&N air filter hoping to gain an extra mpg or so, but found no difference. I have even heard of people getting less mpgs using non factory gimmicks touted as giving you more mpgs. So far I have not read any testimonials from any forum members that any of these gas gizmos actually work as claimed. Anybody got one?
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Old 04-09-2008, 09:08 AM   #33
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Quote:
Originally Posted by BillTex
Rodney, if you look at the curve I sent you, you will see that peak efficiency is just below peak torque on the RPM scale.
Intuitively, you want to think they are one and the same...but another way of looking at it; peak power would require more fuel...
Interesting that your experience (trip log) pretty much agrees with the other oil burners...

Hope this helps.

Bill
Unfortunately, no it doesn't, I remain confused. The figure shows torque and HP curves, but what I do not have sufficient information to interpret is this: how do you determine peak efficiency from that data? That is why I asked if torque was the main issue. Also, the figure you sent shows peak torque at 1,600 RPM rather than 1,800. I'm as lost as last years easter egg at this point.
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Old 04-09-2008, 09:24 AM   #34
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Quote:
Originally Posted by Gen Disarray
Unfortunately, no it doesn't, I remain confused. The figure shows torque and HP curves, but what I do not have sufficient information to interpret is this: how do you determine peak efficiency from that data? That is why I asked if torque was the main issue. Also, the figure you sent shows peak torque at 1,600 RPM rather than 1,800. I'm as lost as last years easter egg at this point.
My bad, the charts shows peak power, the reference to peak effficiency is in the Technology article...

In general, the higher the engine rpm, the higher the output. However, increasing the rpm reduces the combustion efficiency and increases mechanical loss. After exceeding a certain rpm, the effects of mechanical loss become significant, resulting in a sudden drop in output.The brake specific fuel consumption curve shows fuel consumption per rpm. Brake specific fuel consumption is indicated in units of grams of fuel consumed per unit PS per hour (g/PS•h).

Engine fuel consumption depends greatly on the combustion efficiency and often has an inverse relationship with the torque curve. The above figure shows that the fuel consumption is at a minimum (and fuel efficiency at a maximum) at 1,800rpm.

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Old 04-09-2008, 09:30 AM   #35
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Quote:
Originally Posted by Gen Disarray
The speed vs MPG chart has changed over the years (see attached). I wasn't able to find this figure on the EPA site, but it is supposedly EPA data. Notice that MPG loss at speeds above 55 mph has improved since the 70s. I added the arrow at 60 MPH. I wish these figures were available for pickups iin adddition to the combined data for all vehicles.

My current TV (91 F250 7.3L PSD) is fairly new to me, however, it was my impression on the one long trip I have taken it on that it was using less fuel at 60 than 55. I can't prove that as this impression came from watching the gauge, but I wonder if the extra 5mph off set some of the hill climbing issues? Also, this observation was based on a small (under 200 mile) segment of the trip, so other factors such as wind might have been in play. In any case, at 60 mph I was able to get just over 15 mpg towing.

Another question I have is this: what is the best driving approach for fuel economy on the two lane (ie slower speed) mountain roads with lots of climbs and descents? Much of my summer will hopefully be spent towing on these roads so the better the mileage the better.
Which rear end ratio do you have? That plays a major part in it too. FWIW I have a '96 PSD with a 4.10 and my best mileage is right around the 60mph 1800rpm range. I haven another one with a 4.56 and it gets crappy mileage sitting still with the engine off, but will pull the house and everything in it

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Old 04-09-2008, 09:42 AM   #36
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Quote:
Originally Posted by wahoonc
Which rear end ratio do you have?
If I recall correctly its a 3.73
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Old 04-09-2008, 07:01 PM   #37
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Quote:
Originally Posted by Gen Disarray
If I recall correctly its a 3.73
Depending on your transmission that would put your economical "peak" near the 65 mph mark. If I recall with all things being equal you reach peak economy level at the RPM where torque and horse power cross. However, many other things come into play, like aerodynamics...if large square boxes can be considered aero I notice a 2-3mpg difference in my beast just based on whether I have a tail wind or a head wind. Best I have ever done was just over 18mpg over a 500 mile run with a 35 mph tail wind

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Old 04-09-2008, 07:31 PM   #38
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Don't forget to factor wind into the equation.
a decent explanation of how it works in a three dimensional puzzle may be found here, but in general, for our purposes it's enough to slow down some amount roughly proportional to a headwind component, and speed up slightly for a tailwind component.

In other words, it can be calculated (but I haven't the patience to do so) that if your peak fuel economy, towing, is at 60mph, with a 10mph headwind you might do better at 57mph and with a 10mph tailwind you might actually get better fuel economy at 63 or 64 mph.
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Old 04-09-2008, 08:25 PM   #39
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Peak engine & transmision curves have a lot to do with efficiancy, but so does speed. Air resistance (assuming uniform density & temperature) increases as a square of speed. Inclines have obvious gravitational resistance added in, weight becomes a factor on the inclines. My own opinon is that once one gets above something around 35 mph (variable with vehicle) , wind resistance is the #1 drag on fuel economy. Just some thoughts from a basic physics view point.

The glider link was fasinating! Good job.
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Old 04-09-2008, 09:02 PM   #40
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Pointing out that as gas prices rise the focus on squeezing out every last mile out of a gallon also is increasing. This is not a bad thing. But lacking some miracle device that will give a really big boost the amount of fuel saved is still fractional. Even if you add all the tricks and gimmicks together reality is gas to pull your AS is going to cost you.

Not that I don't try the little gas saving ideas. I have a little calculator in my TV to figure mpgs for each fillup, and drive at a prudent speed.

Given that gas prices are high and will only get higher here is a guaranteed way to get at least 5% more miles per dollar, which in the case of my F250 towing at 12mpg average is .6mpg, by doing absolutely nothing. Get yourself a credit card which gives a gas discount. I have a Discovercard which gives a 5% discount. And the more gas you use the more $ you save, right? I recommend my technique of never looking at your credit card statement, let the wife pay the bill. Out of sight, out of (your) mind.
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Old 04-29-2008, 08:13 AM   #41
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My latest econo-run netted 24 mpg from the big-O-Dodge. 664 miles and 27.6 gallons, almost all highway at 65 mph, 20% of the time with the AC on (it seems to drop the MPG-o-Meter .4 mpg).
The overhead meter showed 25.1 to 27.0 for the distance, 26.1 at fillup

I figure that equates to $3.399 a gallon based on my previous 20mpg driving habits.
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Old 04-29-2008, 09:06 AM   #42
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Hi Silver (or should I say Hi Ho?),

Thanx for keeping us posted on the Cummins mileage...they sure are stingy with fuel. Good thing with the way things are going...
I haven't been out in the Chevy in a while, but we'll start running soon as weather is improving here in N.E.

Do you have any numbers for towing mpg recently?

Bill
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