Typically, it's 5-10% more efficient than your foot. The only car I've ever had where it was less was the GSX. I've no idea what exactly what was wrong with it, but the cruise control "surged". You'd set it at 55 and it would cost down to 53 and then surge back up to 57-58. And then it would coast back down to 53 before doing it all over again. The acceleration was rapid enough to build a few lbs of boost, so obviously this wasn't very enjoyable and I really never used the cruise control.

 

Interesting.. The road this was on is relatively flat. Yes, this was in the Merc. This is two round trips and I alternated the directions used with and without cc. The mph seems pretty steady with the cc on.

 

Lol at driving an AMG and worrying about gas money.

 

During a trip back from Texas to Cali, I averaged 35.XXX mpg in my S2K with cruise control on the entire way. If your calculations are correct, I probably would've gotten better than 35.XXX without cruise control... But my legs would've been far more sore than they were with cruise control on.

 

My results are totally opposite. Ive driven 500 miles straight highway at times on cruise, and have gotten 31mpg with cruise....oh yea, my car is turboed. I've hit as high as 35mpg when the car was stock on cruise.

 

The word "money" is not mentioned once in this thread before you posted. What exactly makes you think the OP is concerned with cost savings rather than just being genuinely curious?

 

I don't think 11 miles is really much of a test. Try a tankful on a highway trip.

 

Even if it had been mentioned.... Not trying to be a dick or anything CKit, but just because you have money doesn't mean you shouldn't be concerned with burning it out the back of your car....

Going up to New York a couple weekends ago ~240 miles each way, the way there without cruise, the way back with I noticed little to no difference in cruise consumption. I wasn't keeping anywhere near the level of detail, but just paying general mind, and didn't notice much of a difference. If it were 20% that is significant enough to have noticed from the on board fuel computer. I think 20% is pretty odd.

 

I'm just assuming. Mainly because of the OPs last sentence about modifying his behavior because of his results.

 

No offense taken. I guess it's just a matter of perspective.

I completely agree with the OP that you can get a lot better gas mileage manually driving using hypermiling techniques than you can with cruise control. I understand that the OP didn't frankly hypermile, but even letting the speed drift down and not kicking into a lower gear like cruise control would do... if hilly enough, that could explain the difference.

Sometimes, when we tow... I manually control the accelerator rather than using the cruise. Average gas mileage improves from 10 to 12mpg. That's 20 percent, just by mildly hypermiling the hills.

For the record, I would Lol if I saw someone in a Ferrari hypermiling. I would Lol if I saw someone in a Hummer hypermiling.

I would not Lol at someone in a Civic hypermiling. It's not about me or my money. It's about eeking out efficiency in a car tuned for performance. Kind of like trying to use a steak knife to spread butter, IMHO.

That's just my $0.02.