7 minutes ago, Skodianian said:

I came across this discussion as I was also looking for a way to change acc to a ordinary cc. Reason being I've just changed from 1.4tsi with CC to a new 1.5tsi ACT with ACC. I used to get 53mg on a long journey doing 75. Now with new 1.5 ACT and ACC I get only 42mg at 75mph. I believe it's ACC fault. 

Engine needs more miles to reach its full mpg potential?

If the car in front accelerate and brakes like a hooligan, your ACC will try to mimic that car, making you waste fuel.

 

Also with CC, you can cruise up to a mid-lane hogger and squeeze into a gap to overtake. Whereas with ACC the car will start to slow down as soon as it detect that 60mph hogger, then when you change lanes, you'll waste fuel to get back up to speed.

 

I have found with ACC, it is easier to find a smooth 70mph car and just sit behind them, setting ACC slightly higher than their cruise speed to use them to help you save fuel. I usually get ~55mpg on a long journey, now finding and following a steady eddy helps me to gain 60+mpg during long journeys.

 

As above, ACC will mimic the behaviour of the car in-front.

If they are constantly on/off the Accelerator & brake pedal, then your car will also do the same giving you poor fuel economy.

 

You can try setting the distance to a large setting which gives you more of a coasting gap so your car will be less reactive to changes in-front.

 

At the end of the day CC & ACC are just a system for maintaining a constant speed.

Any good driver will be able to return better mpg driving manually even if the road is completely clear.

2 hours ago, philbes said:

Engine needs more miles to reach its full mpg potential?

I did 300 motorway miles. 

1 hour ago, Skodianian said:

I did 300 motorway miles. 

I meant total miles.  From new an engine needs 1000's of miles running before it reaches its full mpg potential.  I bought my car with 9,500 miles on the clock and the mpg steadily improved until over 18K.

Remember the driving mode setting for ACC will also affect how quickly it reacts to the car in front changing speed. You specify how aggressively the ACC accelerates, and in turn it will affect mpg.

 

 

1 hour ago, philbes said:

I meant total miles.  From new an engine needs 1000's of miles running before it reaches its full mpg potential.  I bought my car with 9,500 miles on the clock and the mpg steadily improved until over 18K.

Fairly typical of VAG engines (and maybe other manufacturers too), my previous car got better mpg after 10,000 miles and better still after 20,000 miles then it stayed steady until I sold it with over 80,000 miles on the clock.