10-4 Magazine - For Today's Trucker

November 2005 - Performance Zone

“CRUISING” OR DRIVING?
By Performance Specialist Bruce C. Mallinson

While eating at truck stops, I often overhear drivers say, “I use the cruise control as often as I can – that computer is a lot smarter than I am.” Or, “I get better fuel mileage on cruise than I do driving with my right foot.” After hearing comments like that, I decided it was time for a little education on using (or not using) your cruise control.

First of all, the computer is NOT smarter than you and it can only try to do what you’re telling it to do. Yes, it does monitor the engine better than your eyes can scan the gauges (the computer scans each sensor every second so if you get a sudden loss of oil pressure the computer will catch it sooner than your eye), but as far as driving goes, your brain and the feel the truck gives you through the seat of your pants, feet, hands, and ears should far outsmart the computer. All that computer knows is that the driver wants to maintain a given speed at all times, and its going to use every once of power it has to do that.

When you’re cruising along on fairly level terrain, watch the turbo boost gauge and how it fluctuates when the truck is on cruise control. If the turbo boost increases by 5 psi because of a slight incline in the road, the engine needs to develop about 100 more horsepower just to maintain the given speed. If you were driving with your foot, chances are you would not have used that much power because it’s not mandatory to maintain an exact speed. By allowing the speed to vary by 3 to 5 mph, the amount of fuel and power needed to go over the slight grade is much less.

While “cruising” on interstates that have rolling hills (like a roller coaster), the cruise control will use 100% of its power to hold the truck’s speed going up the hills, often forcing the throttle to go wide open, which kills your fuel efficiency. Once up the hill, the cruise control might even use the Jake Brake to slow the truck on the downside of the hills. Why allow the Jake to take away free speed or mileage on a moderate hill? Now keep in mind, I’m not talking about a mountain, I’m talking about rolling hills such as I-80 in Western Iowa, I-77 in the Charlotte NC area or I-71 between Cincinnati and Louisville. In areas like these (and others throughout the country) with rolling hills, I would turn off the cruise control and drive the truck with my brain and right foot.

Let’s look at how to properly drive through an area with rolling terrain. Imagine you are heading toward a slight incline. As you approach the hill, gently roll your foot into the throttle and take the turbo boost to about 24 to 26 psi and hold it there as you climb the hill. As you crest the hill, back off of the throttle and then “coast” a bit down the backside of the hill. Just before you get to the bottom, roll your foot into the throttle again and hold it there through the bottom of the hill and up the other side. You will have to decide how much power it’s going to take to get you up the next grade at a reasonable speed. After all, that’s why you have a brain – you can think – the ECM on your engine can’t do that.

You will most likely be a little over the speed limit at the bottom of the hills, however you still have a C.B. radio, get a bear report. Yes, you may have to turn down the volume of your music, but that’s part of driving. If the speed limit is 65 mph in that area, you might get up to 75 mph for a short time in the dip between the two hills. I run my KW around 75 mph on that section of highway and this speed allows my rig to climb the next grade using only momentum and a reasonable level of power. But my foot is not on the floor and the rig is slowing down going up the grades. Then, when I’m near the top again, I back way off the throttle and allow the truck to breathe and cool down. Going across the top of the hill I’m hardly touching the throttle, and going down, I’m mostly just coasting.

If I had allowed the cruise control to drive my truck, it would have used every bit of horsepower the engine could develop to get up the hill. If you can pull a grade using 5 psi less turbo boost, you are using about 100 less horsepower. Pay attention to the turbo boost gauge. In fact, you should mount one up on the dashboard right in your line of sight and drive by it. That $45.00 gauge will save you a tremendous amount of fuel.

If you happen to be on I-95 heading to Florida and the only grades on the highway are the overpasses, leave the cruise control on. About 1/4 mile before the overpass, roll your foot into the throttle, increase the turbo boost by about 8 to 10 psi, and hold your foot there until the top of the overpass. Try this and see if the engine uses less boost than when the cruise control is doing everything.

With all this said, you probably think I do not like to use the cruise control. Truth is, I do like to use it. In fact, I use it quite often when driving my Cummins-powered Dodge pickup, but I will not allow the cruise control to pull my throttle wide open when pulling a trailer (or even empty). Driving my Kenworth, I only use the cruise control on the level terrain. Once the turbo boost gauge starts to vary by 3 to 4 psi, I override the cruise control with my right foot.

While recently speaking with an owner-operator team that drives 270,000 miles per year, I was disappointed to hear them say, “We can’t be bothered driving the truck with the throttle.” Do you want to drive the truck or do you want the truck to drive you? This team could save $13,750 in fuel if they improved their mileage by just 1/2 mpg. Isn’t that savings worth a little effort?

For more information about high-performance diesel engines or for other useful driving tips, visit www.dieselinjection.net or call Bruce at Diesel Injection of Pittsburgh at (724) 274-4080.