The "Sweet Spot" (best MPG)

Our KW runs up to 2000 rpm. Would that we always had a 30 mph tailwind...

 

I'm continuing to learn how to drive efficiently. Until I started this thread and started to read some of the posts from other truckers (and read other threads) I'd been driving as my brother (the O/O) had been driving, with cruise control set at 67-71 mph in our 2003 T2000 KW with a D15 and 13 spd transmission. Typically our mpg was in the mid to high 5's, sometimes in the low 6's. Needless to say our higher speed was the issue, and I think his concept to keep rpm's at 1500-1600 in order to protect the engine is not correct.

By dropping speed to 61-62 and rpm's down to about 1250-1300 I think this is really where we had to be. Recently I was able to finish two runs where I was the only driver from fuel stop to fuel stop that has me very excited:

1. Running from Winslow, AZ to Lake Havasu City, AZ westbound on I-40 with little wind. Granted, this is a net loss in elevation. However, I posted an mpg of 6.986... a record for our truck. Most of this was done "free footing" (without cruise control).

2. Oklahoma City, OK to Sullivan, MO - eastbound on I-44 - little wind. This has a lot of rolling terrain. Most of this run was done free footing. Only three times did I ever let the boost gauge get above 75% of maximum boost when pulling any of the hills and one of those times was in order to keep above 65 mph in order to allow trucks behind me to not get stuck. Most of the hills I kept boost below 50%, allowing speed to bleed off (sometimes to 50 mph) and had to downshift from top gear a handful of times. MPG was 6.875! By comparison the first time I ever ran this section and "owned" the tank of fuel (running the old way) my mpg was 4.3! A couple weeks ago, as I was learning this new way of driving, I had run 6.03 mpg (with a slight headwind).

I think cruise control is best used on very flat terrain. Any kind of rolling hills and I prefer going without cruise control in order to "defend" the mpg and reduced boost rather than defending speed. No matter what, this is a much more fun way to drive. It's also fun to get to the fuel pump to see how little you need to pump!

By the way, we will be arriving at our delivery well in advance of schedule. Therefore there isn't any need to put "the pedal to the metal" (unless we have to catch any critical reruns of NCIS at the truck stop...)...

 

Thanks for the kudos, double yellow.

Laytonrock, I'd also considered cruise control as better than free footing. However, as I've started free footing more and more, I'm now of the opinion that cruise control can't match mpg when it comes to rolling terrain. My experience earlier today is an excellent case in point, when compared to my previous runs on I-44 in Missouri.

The first run I was running "the old way", with cruise control set at about 70 mph (about 1600 rpm) and getting 4 mpg. The reason we set the cruise this high is so cruise control could climb the steep hills in top/top gear. However, this meant that boost would quickly climb to maximum (28 lbs) and mpg would drop to 2.7 mpg. Considering the long stretches at 2.7 mpg it is difficult to "win it back" on the down hill runs, especially when cruise control continues to keep maximum boost after the top of the hill to get back up to 70 mph when we topped out at less than 70 mph.

Free footing today I would try to top out on the hills in top/top gear at around 55-60 mph and if necessary downshift to top/low split and top out at 50 mph. I am starting each climb at 65-75 mph, applying about 8-12 lbs boost (4.5 to 6 mpg) to begin each climb and gradually easing up on the throttle as I climb in order to maintain at least 4 mpg as I climb. I'm allowing speed to decline gradually as I climb the grade. On grades that aren't very steep I would often find an equilibrium point, where I could maintain 4.0 to 5.5 mpg as I climbed at about 1200-1300 rpm. Cruise control would never be able to do this because cruise control is programmed to maintain a minimum mph and not a minimum mpg.

Now, even on very slightly rolling terrain, like I-40 in west Oklahoma or in New Mexico, I won't use cruise control. Over the last couple of weeks many times in these sections I've switched back and forth between cruise control and free footing, and find that CC will almost always push down mpg as it tries to "defend" the minimum speed. I find that I can ease off the throttle, maintain a higher mpg, and frequently will have almost no decrease in mph on very moderate rolling terrain.

Free footing certainly requires a lot more concentration. I'm constantly monitoring mpg, boost, and listening to the pitch of the turbo whine. I'm getting to the point now where I can tell where I'm at just by the pitch of the turbo, with an occasional peak at the mpg and boost gauge.

I'm having fun with this. Can't wait for the next session where I "own" the tank of fuel to see whether I can break the 7 mpg barrier in our old truck (1.1 million miles without a rebuild...).

 

I did an analysis of the cost savings per thousand miles using cruise control "the old way" at 70 mph versus "free footing" and allowing speed to decline on the hills.

View attachment Analysis of MPG.pdf