Honiton-based Bale Group, which runs 55 lorries and employs 120 people, said it would save £25,000 a year in fuel.
Widening the road would provide an alternative route to using the M5 and M4 but campaigners say the plan would be devastating for the environment.
The views of businesses will be presented to the government later in September.
The A303 is one of the most direct routes from London and the South East to Devon and Cornwall, but single carriageway sections, including around Stonehenge, suffer from bottlenecks.
Traffic also builds up after accidents on the alternative M5 route into the South West.
Tony Bale, managing director of Bale Group, said: "We get two or three miles per gallon better fuel consumption by going on a dual carriageway than we do by going on the normal A roads, especially the A303 because you have traffic jams and everything else that goes with it.
"The cost to us is about £25,000 a year extra if we go that way.
"That's not very good for the environment, your carbon footprint or anything."
Environmental campaigner Andrew Bell said rail should be improved and a wider road would threaten the Blackdown Hills.
He said: "It's utter madness. This is an Area of Outstanding Natural Beauty and to put a dual carriageway through it would be devastating for the environment."
Conservative MEP Giles Chichester is backing proposals by Somerset County Council to dual the road.
He said: "Dualling the A303 all the way to Cornwall would improve the environment. If you make traffic smooth running there are less fumes."
The A303 is 92 miles (148km) long and runs through five counties.
The proposals, which also include upgrades to the A358 and A30, are being developed by a multi-agency task group consisting of Highways Agency representatives and county council officials from Wiltshire, Somerset, Cornwall, Devon and Dorset.
A Devon haulage firm has backed calls to dual the entire A303 from Exeter to London.