CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

The math of cruise control

so here’s a car math question.



If you travel on roads that are mostly up and down hills, if you use cruise control it will be constantly adjusting.



in terms of fuel efficiency (or any other issue like “wear and tear”) is it better not to use it or is it still better and more efficient than a human being making those adjustments?

Too many unknowns to use math. A gentle-driving, conservative, well-informed human can probably do better than cruise control. The more variable the conditions, the better the human could do (cruise control cannot look ahead on the road to anticipate what to do).

But how good is the particular human doing the driving? From what we see on the roads, maximum likelihood estimator says “not very good”.

For fuel efficiency you will do better by not using the cruise in hilly terrain. I had this question long ago with a motorhome; tried it both ways. Cruise will force your vehicle to maintain a set speed at any fuel efficiency cost. If you duplicate this with your foot, I would expect similar results.

For best fuel efficiency you can permit the vehicle to slow a little when a hill is encountered so the engine does not work as hard. Alternately, you can gradually speed up a little anticipating a hill and then permit your speed to drop a little when the hill is encountered. This might use a little more gas than simply permitting your speed to drop on a hill climb but less gas than cruise.

that was sort of the assumption, because the cruise control has basically zero tolerance for variation, whereas if no one is behind you to get annoyed, why not let it vary a couple of mph on the hills and save some fuel.

If programned parameters are not exceeded it will not be constantly adjusting.

If you say I am driving on a road that I know that the cruise constantly adjusts you need to tells us if these adjustments cause the car to good to full power (with auto trans downshift).

If your situation is that the trans downshifts when using cruise but when cruise is disengaged you can manually maintain speed without trans downhift, switch off cruise and go manual for better mileage and less wear and tear.

As said above, the cruise control operates on a PID loop. Proportion, Interval, Duration. X amount of deviation from set point it will react a certain amount of throttle opening. If the RATE of deviation from set point is accelerating, it will increase the throttle opening even further. If the deviation from set point is too long, it will then add more throttle input.

If you’re on a relatively flat plane and the mph drifts +/- 1-3mph …but doesn’t do it long enough or fast enough, it won’t react.

In cyclical hilly terrain it works best to pace yourself on a mean speed …accept the slow down on the up hill and take the over run on the down hill. It works here in the “rolling hills of Pennsylvania” …lengthen the duration out and you lose that “gravity acceleration” to compensate for the other end of it. Keep in mind that depending on your fuel strategy at the moment, you are using no fuel under no throttle.

It is best not to use cruise control on hilly roads. Safer, and better on the transmission.
Also you should lock out the overdrive.