MPGs when idling / rolling down a hill

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Thu 11 Dec 2008 01:31
MPGs when idling / rolling down a hill - far0n
Citroen C4 Picasso 7 seater

Right, if I dip the clutch down a hill the mpg counter gives roughly say 400mpg, as you would probably expect. Strangely though if I 'engine brake' it down the same hill it goes all the way up to 999mpg. Now I saw something a few weeks back that suggested modern engines when you take the foot off the accelerator stop the flow of fuel completely when going downhill.. Is that true ? It would certainly explain what I've noticed !

{made non make/model specific}

Edited by Dynamic Dave on 11/12/2008 at 10:17

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Thu 11 Dec 2008 06:41
MPGs when idling / rolling down a hill - Ben79
Yes that is true! Most (if not all) cars produced this millennium cut the fuel completely when lifted off the throttle. Fuel does begin to flow before tickover is reached to prevent stalling.

I haven't tried on Dad's C4 Picasso but on my C5 the mpg meter reaches 999mpg faster if you tap the brake after lifting off the throttle. I think it cuts the fuel supply quicker.

Driving down hills with the clutch engaged prevents the car from running away and helps prevent overheating the brakes through constant use.
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