Does using cruise control really help you save gas? Tom and Ray settle a bet.

Dear Car Talk

Dear Car Talk | Dec 01, 2006

Dear Tom and Ray:

My gentleman friend and I have a steak dinner bet riding on your answer to this question. I say using the cruise control on our highway trips saves gas. It saves gas because I am not bopping between different speeds, just going at one steady speed. He says this uses more gas. He adds that there is some component that actually uses gas to run the cruise control; therefore, we are using more gas! I think that sounds a little nutty! So tell us ... who will be ordering the perfect steak? -- Christine

TOM: You'll be ordering, Christine, and he'll be buying.

RAY: Using cruise control on the highway does save fuel, for exactly the reason you say: It keeps you moving at a very steady speed. Continuing to move at a steady speed uses less fuel than accelerating.

TOM: When you drive without cruise control, you tend to slow down, speed up, slow down, speed up, etc. And each time you have to haul that 3,500-pound hunk of steel back up to speed, you use more gasoline than you would have used if you had just kept it cruising steadily.

RAY: Plus, cruise control keeps you from accidentally going too fast -- from looking down and suddenly noticing that you're doing 80 -- which is the biggest waster of gasoline. Because wind resistance increases by the square of your speed, wind resistance increases by about 50 percent between 65 mph and 80 mph. That takes a HUGE toll on your mileage, because your engine has to push your heap through that much more wind.

TOM: To address your friend's other point, Christine, the cruise-control system is run off of engine vacuum and electricity. There's no cruise-control system that runs off of gasoline. So have him buy you a bottle of Ravenswood Sonoma County Old Vine Zinfandel to go with your filet mignon for that boneheaded suggestion.

RAY: We should add that in the old days, you could make the argument that cruise control could occasionally waste fuel. For instance, let's say you had it set to 65, and you cancelled it by stepping on the brake and then slowed down to 45. When you turned it back on, by hitting "Resume," older cruise-control systems would simply floor it until you were back up to 65. That hard acceleration certainly wasted some gas. Newer cruise-control systems, in contrast, are smarter and accelerate much more smoothly.

TOM: But even with an old system, unless you were turning it on and off frequently, you'd still come out ahead because of the steady speed you'd maintain on the vast majority of your trip.

RAY: But if you want to throw your gentleman friend a bone (after you've finished eating the rest of the steak), you can tell him that under certain circumstances, older cruise-control systems could waste some gas. But, generally speaking, you are completely correct, Christine. Buen provecho!

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