Cruise Control - uses more fuel?

[Doug-SRidirect]

Just a thought, but I’m certain my Vectra uses more fuel when the cruise control is used extensively.
My theory: With no cruise control, on motorways you tend to speedup going downhill and slowdown going uphill, witness the positioning of speed cameras often towards to bottom of long hills. In a subconscious effort we are throttling back going up-hills and letting the car gather its own momentum going down-hill.
With me so far?
Well, using cruise control, the throttle opens as much as necessary to maintain the constant speed uphill (on my auto, it even changes down a gear sometimes) and to make matters worse, it closes the throttle completely going downhill in an attempt to prevent the car’s speed from rising – thus wasting all that potential energy. You’ve gained no speed going downhill and as soon as you hit the bottom and start ascending, the throttle opens, and it powers you for the full length of the hill.
Maybe my imagination but for the last couple of weeks I’ve not used the CC on my M1 commute and think I’m getting a couple of extra mpg overall. More likely it’s my imagination, but it’s a thought nonetheless.
This is just a theory and I’m not so tight as to let it change my driving style, I’ll be back using the CC next week….and hang the expense!!!
Any thoughts anyone?
Doug

[Ozz]

I have been thinking the same for a while now. To confirm one way or the other I reset the trip computer thingy & covered a certain amount of Km's using then not using the CC. I made sure the car was carrying the same amount of added weight & fuel. At the end of it all my fuel consumption dropped using the CC as oppossed to not using it

[Harry]

i've been wondering the exact same thing, i've not tested whether or not this is true but my theory is as follows:

doug -> i mainly agree with you, except for the bit about travelling down. i don't think you are wasting the momentum of the car because the engine will cutout unless it is required to accelerate and therefore your trip computer screen should read 999,99 instantaneous MPG, hence increasing the MPG figure.

One thing i don't understand is why the instantaneous MPG reading is greater when the car is in gear but with no load on the engine i.e. decelerating or driving down hill, as opposed to when the car is in neutral and coasting, where the instantaneous MPG will show approximately 70 MPG and vary drastically. Anyone got a clue?

I think overall the CC will reduce the MPG returned though.

[SCOTTEX]

I'm at a loss how you can use your CC on the M1, its all slow, stop, slow, stop.

[kitchpoo]

CC is useless on inclines and descent as you are right - it doesnt foresee the advantage of carrying momentum through, also on twisty roads it can make the car unsettled as it decides to load power on or off mid bend, living in the flatland - it is fine and usable most of the time though. I use the CC like a throttle and will click it on and off all the time, off downhill descent - and then back on uphill (thats why my 3.0 CDTi gets over 50mpg!)

[Doug-SRidirect]

I start work at six am!

On the overrun the fuel is cut off totally but when it's ticking over the tick-over jet (or whatever) opens and it uses more fuel!

[Harry]

kitchpoo ->50mpg!!! i'm gobsmacked!

have you installed a diagnostix chip or similar? were you still getting such good mpg before the power upgrade?

[kitchpoo]

before the pbox it was about 50 , now its about 51, a small increase.

[Tim2100]

One thing i don't understand is why the instantaneous MPG reading is greater when the car is in gear but with no load on the engine i.e. decelerating or driving down hill, as opposed to when the car is in neutral and coasting, where the instantaneous MPG will show approximately 70 MPG and vary drastically. Anyone got a clue?

Yes the car will use zero fuel when in gear but no load as the moment of the car will keep the engine spinning, modern engines need no fuel to keep turning, whereas if you are coasting out of gear it will have to use a small amount of fuel to keep the engine going on tick over.

Tim.

[Harry]

i see...thanks for clearing that up for me Tim.