A £4 million budget has been earmarked to introduce the additive, called Eco-tab, into the UK market.
The agency, which won the business in a pitch against BMP DDB, is currently working on a campaign to raise awareness of the product which can save an average family £360 a year on fuel, according to its manufacturer.
The advertising will begin with a regional test in January to be followed by a national roll-out later in the year. If successful, Eco-tab will be launched in mainland Europe.
Eco-tab is a tablet which is dropped into the fuel tank. It was first sold in the US five years ago and is currently distributed in more than 20 countries, including Australia, Japan, Singapore and Canada, under local brand names.
A UK operation was established in November last year. Dee Baker, Eco-tab's managing director, said: "Our aim is to become a leading player in the market for consumer automotive products in the UK."
Despite global differences in fuels, vehicles and operating conditions, Eco-tab is said to be able to reduce fuel consumption by up to 10 per cent and emissions by up to 50 per cent in either petrol or diesel engines.
Nick Howarth, the HHCL/Red Cell managing director, said: "Eco-tab offers increased car performance while reducing fuel consumption and harmful emissions. It's a real win-win for consumers and we're really looking forward to contributing towards its success."