Around 80% of households are currently enduring "transport poverty", according to the RAC Foundation.
People in this situation are being forced to spend 10% of their disposable cash on public and private transport, the foundation said.
Among the top expenditures were buying a car and covering running costs, such ascar insurance , road tax and petrol.
Out of the lowest-earning households around 9% of disposable income is spent on transport, while the highest-earning fork out 15.5%.
RAC figures indicate that average car and non-car households use around 14% of their weekly expenditure for transport, which is typically £64.90 being spent out of £473.60.
RAC Foundation director Professor Stephen Glaister said: "Rightly, there is much concern about the four million households who need to spend more than 10% of their income to keep warm.
"Yet this figure is dwarfed by the 21 million households which spend over 10% on transport. For the average household, transport is the single biggest outgoing, bar none."
He went on: "The situation is even starker when you look only at those homes which have a car or van. In these cases, the poorest fifth of households are spending at least 17% of income on a vehicle - leaving aside anything extra that goes on public transport."
Copyright Press Association 2012