Hi All,
I use Rich’s technique all the time also. The Latch mode (and a few other settings) let this happen naturally. Ken is quite right; our drag coefficient (CD) is probably on the order of .6 or higher for those aeronautically inclined members. This compares to something like a Buick Regal with a CD of probably less than .25. As speed increases, the drag goes up as to the square of the speed. So, for instance, the difference in drag going 60 verses 65 is an increase of approximately 17% or a decrease in fuel efficiency of say 20% when additional frictions are added like increased RPM and tire rotational speed.
As should be apparent, headwinds are a MPG killer. As an example, lets assume a speed of 60 MPH with a 10 MPH tailwind verses a 10 MPH headwind. The other drags will be about the same since the engine RPM and tire rotational speeds are the same. The headwind will increase drag by 96% over the tailwind. I keep telling my wife that we should never plan a trip; just see witch way the wind is blowing that day!
Hi Suncircles & Ken,
Pilots say “The net effect of the wind is to slow you down” or in our case, “The net effect of the wind is to lower MPG”. I too have gotten lousy MPG coming west on I40 into headwinds. The worst was 7.2 MPG. This is somewhat made up for by the occasional tailwind. The best we have ever had was a tank full that worked out to 10.35 MPG. Our engine computer via the Palm Pilot and CAT ‘s Pocket Tec software (no longer available) is reporting 90,914 miles on 10690 gallons of diesel for an overall average of 8.50 MPG. Bear in mind that when new from the dealer in Tucson, the computer read 6.3 MPG. It’s taken this long to get it up there but I’m quite happy with its performance now. Idle fuel is another issue as well as generator use. I use a figure of .33 gallons per hour for the generator and subtract that from the total fuel purchased to get a more accurate MPG. The engine computer also says I have used 13 gallons of fuel idling.
Scottma,
To answer your question, the ‘Latch’ mode leaves the engine brake off when you let off the throttle and it only comes on after you press the service brake (or hold down the ‘cancel’ button) for a set length of time. (.5 seconds works well) This let’s you coast down hills without the brake coming on. There is another setting involved that determines at what speed, over that set for cruse, that the engine brake will finally come on. I have mine set to 15 MPH, which leaves the brake off on all but the steepest and longest hills. Since I set 58 or 60 MPH in cruse, that means the brake will come on at 75 MPH out on a freeway.
Other operations are changed also. Of course, when the engine brake ‘switch’ is off the engine brake is off. When the switch is on and the cruise is off, the brake operates in either the ‘Coast’ or the ‘Manual’ mode. You can tell which by the way it works. With the switch on, in ‘Manual’ mode, the engine brake comes on whenever you let off the throttle. With the switch on, in ‘Coast’ mode the engine brake comes on as soon as the service brake is pressed and off when the service brake is released. When the switch is on, and Cruise is on, and ‘Latch’ mode is set, then it works as described above. This prevents needlessly slowing down when you just tap the brake to release the cruise control. Not slowing down so much means not having to accelerate to regain the speed. Ergo less fuel is used. You know, those times when you see a light change to red way up in front of you and you want to just let off the throttle and not use either the service or engine brake to save fuel and reaching to turn off the switch is more trouble than it’s worth. ‘Latch’ mode does just that. And of course, as soon as you step on the accelerator, the engine brake turns off if it was on.
Another setting to make sure is turned on is called ‘Soft Cruise’.
I think you will find that the best fuel economy will occur at that set cruise speed that just lets the transmission drop into 6th gear and stay there at least on little slopes. This assumes that that speed will be somewhere around 55 to 60 MPH. The C7 is most efficient at 1440 RPM producing enough torque (read Horse Power) to maintain speed. As RPM goes up, MPG goes down, because of friction losses in the engine. If your couch drops into 6th gear at just under 1500 RPM and 57 MPH, then you are set up to get the best MPG. Dropping your speed from 65 to 60 MPH should increase your mileage from 7.2 to close to 8.5 MPG. That Land Rover is the rest of your lost fuel efficiency.
Any CAT dealer can reset the computer with the appropriate settings. If you can find someone with a CAT Pocket Tec, then it’s very easy to read out the data and reset whatever you want including the speedometer calibration. It’s nice to have it dead on. If you are ever in Albuquerque, I have the cables for both the CAT 3126 and the CAT C7. Just look us up at Enchanted Trails RV Resort at the top of 9 mile hill west of town. Space C1, until we move to Mississippi of course.
