Likes to tow wrote: I find the best MPG is obtained at the slowest speed at which it does not come out of overdrive. Overdrive kicks in at 45 mph. For those that think the sweet spot is different on different vehicles I just find that hard to believe. Wind resistance plays a major role in gas consumption! Wind drag increases at an exponential rate the faster you drive!!! So try a tankfull running at 50 Vs. running at 70 on A N Y vehicle and you will experience a difference!!!
Why this nation cannot figure this out is beyond my comprehension. We are too spoiled and set in our ways to slow down. Driving fast is nothing but a bad habit.
Do you find it hard to believe that rear axle ratios and tire diameter will affect the engine rpm at any given speed? If you had 2 trucks with the same engine, transmission, and tire size but one had 3.55 and the other had 4.10, which one do you think would be more economical at 65 mph?
The 3.55 but the difference in engine efficiency at the two different RPM's will be minimal compared to the amount of power required to overcome rolling and aerodynamic forces at the higher speeds. Going faster requires more power. More power requires more fuel. It really isn't much more complicated than that.
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pupeperson wrote: Rubiranch said: "At that speed in my car it's turning about 800 rpm..."
In a direct gear? Lets see, if that were accurate and your cars engine would turn 4,000 rpms (relatively low by todays standards), it would go 180 mph in direct (assuming it had the power and wasn't electronically limited) and probably still have overdrive to go. Nice car!
Thank you.
I was wrong, that's in overdrive. The car will actually go faster in 3rd than overdrive (power or a lack of). It's only been up to 140 (138) once in it's life and that was back when it was new.
I'll see today what it does at 40 in a "direct drive".
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I second the Scan Guage 2 comment. I added the info for horsepower output and found on my last trip my truck mileage towing was relatively the same 50 mph up to 58 going north on my last trip. Once I got above that speed mileage started to drop off. It took an average of 75hp to tow at that speed and just one mph higher required 80 with cruise control. Increased hp equalls decreases mileage. Return trip I had to stay at 55 mph to keep the same mpg. I guess a slight headwind was experianced on the return trip. Scan guage 2 highly recommended if you tow alot and have the time to slow down to your best economy while staying safe. Will not work if your vechile is older than 1996.
Let's see... Since it's common internet forum knowledge that you get your best mileage at peak tark rpm, hmm, 4200 peek divided by... carry the nine, pie are square of the third side of an aposolized triangle minus the negative integer of seven... So, I get my best mileage in O/D at 4200rpm and 153mph!
Reality is, 40~mph seems to be the speed my overhead likes the best. Must be the speed when forward momentum overcomes the wasted hp required to run my a/c and a/t along with rolling resistance. From there up, it takes more hp to go faster. More hp means more fuel regardless of what rpm you're running. My Cummins powered Rams were the same way. The faster I went, the more fuel it burned.
I sure don't look forward to that 55mph national speed limit that the Republican senator is suggesting.
One more thought. On the Harley you can really feel what the scan gauge is saying. Up to 55-58~mph, the wind is a gently breeze. At 70mph, it's literally almost hurricane force winds. I can't imagine the wind resistance of our truck and 5th wheel at 70mph.
'05 2500HD Dodge 5.7L Hemi 5spd auto/3.73 SLT. 95k miles
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Like everyone says, "it depends", and it depends on some complex interactions between a bunch of competing items.
Wind resistance increases as the SQUARE of speed- thus, speed increases hurt your wind resistance, extra bad as you go faster.
On the other hand, driving 5 MPH won't do it because you are in low gear and mostly spinning the engine, and expending energy to have tons more RPM/mile and may even be wasting energy on sucking air through a closed carb, so by this rule, you have to go fast enough to hit high gear.
So, while the gears get better, the wind drag increases exponentially and there is a "sweet spot" out there where the magic lines criss cross.
But engine sizes, gear ratios, and draggyness of the rig make those lines different on every set up.
I do like the fancy GM computer systems that give you "intantaneous" MPG readouts....if you can find a perfectly flat road, you could experiment to find the particular "sweet spot".
There is an important factor to also consider when looking for that "perfect speed" that yields the best economy. Yes, different vehicles are different, so there is really no justification in trying to regulate speed for economy, but here is the most important reason of any!
Out here on the east coast, our roads are running at or near capacity. The fact that the speed limits are relatively high in certain areas means that they move large volumes of traffic quickly. Regulating down the speed limit would in essence cause MORE fuel to be consumed because it will reduce the capacity of the highway. It's called the bottleneck effect, and it's produced when more traffic enters the highway than leaves it. Bring down the speed limit in one area (where people are exiting the highway), and you cause a traffic jam (where more people are entering the highway)in another!
that's in overdrive. The car will actually go faster in 3rd than overdrive (power or a lack of). It's only been up to 140 (138) once in it's life and that was back when it was new.
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