The trick to maximizing fuel efficiency on the highway is to minimize the variance in throttle opening and thus fuel use.
It probably is. I say that because there are qualifiers. It depends on the driver and the cruise control systems.
The trick to maximizing fuel efficiency on the highway is to minimize the variance in throttle opening and thus fuel use. A driver who is paying strict attention and looking far down the road can anticipate issues that will require a change in throttle opening and make gradual and slight adjustments accordingly. A hill will require more throttle while a traffic situation may create a need for a lower speed. A cruise control system cannot see hills and traffic flow. It will maintain speed until the driver steps on the brakes and use the necessary throttle and fuel to resume that speed when required.
But there is a vast difference in cruise control systems. My regular test route contains a variety of elevation changes, some pretty steep and sustained. I have seen cruise control systems that allow a variance of more than 10 per cent, allowing the speed to drop that much before taking corrective action. When this does occur, it requires a pretty significant dose of throttle to get back to speed. Other systems will tackle these same grades allowing no more than a 1 per cent change in speed.
Another issue in this respect is the number of gears in an automatic transmission. At highway speeds, the first thing a cruise control system will do when faced with a hill is unlock the converter, allowing the engine to rev up and generate more power. If that is not enough, it will tell the transmission to shift to a lower gear. If there are eight speeds in the transmission that might mean a 1,000-rpm increase in engine speed. If there are only four speeds, that might mean a 2,500 or greater increase in engine revs and fuel use.
Generally speaking at highway speeds, on relatively flat roads with little change in traffic speed, cruise control will result in better fuel mileage.
DOES CRUISE CONTROL SAVE GAS?
Because I am not the most mechanically inclined person alive, I turned to a popular, well-regarded source for automobile information. The Internet. Seriously, what can’t you learn about on the Web these days! A quick search of Google yielded a few promising results, but one in particular stuck out. Edmunds.com, the popular website for automotive consumers, had this to say in an article on tips for improving fuel efficiency:
“Using cruise control can improve your gas mileage by helping you maintain a steady speed, but only if you are driving on mostly flat roads. If you are driving in hilly terrain, using cruise control typically causes your vehicle to speed up faster (to maintain the preset speed) than it would if you were operating the accelerator yourself. Before you push that cruise control button, think about the terrain ahead.”
Makes sense to me. In fact, one of the things I noticed when pulling the few hills I encountered along the way was the car held back a little on the downhill and lost momentum, requiring more gas to pull up the hill that followed. Had I disabled the cruise control I could have simply allowed the car to gain some speed and then coasted up the first half of the next hill. Careful, a speeding ticket here will wipe out any potential savings!