Lets suppose your car has a 3.0L V6 and 10.0:1 compression ratio
then you have 300 cc that must be filled with air/gas mixture about an optimal 15:1 to ignite reliable and cleanly.
But during traffic jams or stoplights, it can switch to 30:1 compression ratio.
Thus you have only 100 cc to be filled with explosive mixture.
Of course an 3.0L engine at 30:1 compression ratio would guzzle more gas than a normal 1L engine at say 10:1 because of larger friction and pumping losses.
Do you think variable valve timing would be needed at all for significant consumption improvements?
Variable compression ratio - Wikipedia, the free encyclopedia