Ford says the Focus 1.0-litre EcoBoost will achieve class-leading fuel efficiency of 4.3 l/100km (67.5 mpg).
Equipped with a specially calibrated 100hp version of Ford's award-winning engine, the new Focus 1.0-litre EcoBoost also will feature Ford ECOnetic Technology.
The car features ultra-low rolling resistance tyres, unique under-car aerodynamics and revised gearing to help deliver the company's most fuel-efficient ever petrol-powered family car.
"Even just a couple of years ago, few people would have thought it possible that a medium-sized petrol car could break the 100g/km CO2 barrier," said Barb Samardzich Vice President Product Development, Ford of Europe.
"The 1.0-litre EcoBoost engine has a habit of taking expectations of what a petrol engine can deliver and turning them upside down."
Ford says it will go on sale early next year and in Ireland will be in motor tax band 'A2', which attracts a rate of €180 for annual road tax.