Guys really wondering cruise control saving petrol on long journey on the motorway? 

The short ansewr is no, it reacts quite slowly to changes in elevation so ends up using more fuel, but to concentrate on driving efficiently for a couple of hours is hard so, the benefit  may show over longer distances and when you are not feeling 100%.

Also a lot depends on your driving style, most drivers arnt fuel efficient, so you may get a good result from cruise control.

I asked myself this question when I first got cruise control.   I often use the M1 so I did a few journeys without CC and a few with.   I found that using cruise control decreased my fuel consumption by an average of 2 mpg.

If driving without cruise control, to a certain extent you speed up down hills like taking a run up for the next hill and then as you go up the next hill you slow down a bit gradually.  I think cruise control would be worse as it won't take the run up and it will use more throttle going up hill to maintain speed.  If on flatter roads I suspect that cruise control would not waste as much fuel as it would with hills.  This is a guess by me - sounds logical?

I'd agree with Isetta, if you live in a fairly flat area the CC uses just enough fuel to maintain speed and won't keep creeping up as your foot gets tired and leans on the pedal more so should give better mpg.  If you live in a hilly area, most of us accelerate before the hill, but the CC won't until it feels the extra load on the engine, so uses more fuel to climb.  

I use cruise control all the time tbh, I've fitted it to a few cars so have a direct before and after comparison and it hasn't made any difference over a tankful for me.  I don't use it for economy though, I use it for a much more relaxed drive.  

On motorways it definitely saved me fuel...as did the speed limiter...

I'll leave the reasoning for that to your imaginations 😂