LibertyCRD wrote:
I respect you guys. You are true diesel enthusiasts. I love you guys.. sniffle...I really do.
But it really baffles me how you can get better fuel economy at 75 or 80 miles per hour than you can get at 55. That totally defies the laws of physics, and goes against the good common sense we all have.
Like I said...I respect you guys and I'm not flaming anyone here. But I am really trying to figure out how you guys are doing it. The faster you go, the more wind resistance you meet, the more fuel your engine drinks. It's a lot more than RPM and formulas.
3 possible differences here that may account for it. Myself (and oldnavy too I assume) are still running OEM tire size, no lift kit, and using biodiesel. From your two posts and sig, you're running different tires, a lift kit, and running straight #2 diesel.
Tire size - could be that the different size tires have changed your engine rpm's at certain speeds enough to throw you out of the "sweet spot" at anything above 65 mph, or if they're larger and heavier tires, the engine is having to provide just enough extra oomph at speed to rotate them that it's just enough to throw fuel mileage off. Also tread pattern, wear rating of the rubber compound (hardness), air pressure, how well they're balanced, alignment, can affect rolling resistance which can have an effect on fuel mileage.
Lift Kit - raising the CRD may have altered the aerodynamics such that they, well, flat out suck at higher speeds. Generally, higher up off the ground means that much more air going under the vehicle and buffeting around on all those irregular surfaces underneath, creating turbulence and drag. Some of the people I know that are into electric vehicles will go as far as bellypanning the entire underside of their vehicle and installing a front air dam just inches off the ground, and it did make a large difference in the amount of power it took to maintain a certain speed and the range they could achieve. Generally, allowing for the vehicle itself, at speeds over 40 mph wind resistance does become the overriding factor.
Fuel - could be the difference between low cetane winter blend #2 and relatively high cetane B20 biodiesel. We've already hashed this one before, quality of fuel makes a HUGE difference in this beast's performance. B20 and TxLED Diamond Shamrock gave well over 30 mpg on the interstate, Citgo and Exxon were lucky to get low 20's on my recent trip.
And it could be a combination of all the above, plus all the other unknown variables such as terrain, etc.
One other thing to remember, our diesel is basically twice as efficient as a gasoline engine, as well as having different HP and torque curves. From what I've seen playing around over the years with gasoline, electric, and now diesel vehicles, the more efficient a vehicle is to begin with, the effects from small changes in things such as wind resistance, tire pressure, alignment, etc will be larger and more noticeable. On an electric truck I have, switching to Red Line synthetic oil in the tranny and differential netted me a 25% reduction in the amount of power required to drive 20 miles to work during cold weather, actually reduced it below what it took during the summer with 90W dino oil. That truck weighs in at 3500 lbs with all the batteries, but the electric motor is anywhere from 85 to 95% efficient depending on conditions. I then did the same with my Subaru wagon, 2000 lbs four banger gas engine straight drive. No noticeable difference at all, any beneficial effect the synthetic oil may have had as far as fuel economy was lost in the noise because that gasoline engine was so bloody inefficient to begin with, it would have taken a major improvement to have any effect.
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