The retailer said like-for-like sales fell 2.8 per cent in the 13 weeks to 30 September, compounding a 1.1 per cent drop in the previous three months.
Sales of car maintenance products were down 3.3 per cent.
However, this drop was partly offset by a 5.7 per cent rise in bike sales as people took up cycling to work, while the recent success of British cyclists such as Mark Cavendish helped boost premium racing bikes.
The retailer has warned that gross margins will be down by one percentage point this year following a series of promotions aimed at triggering a rise in sales.
Half-year profits will be between £53m and £55m, down from £68.7m last year, the company said.
Chief executive David Wild said: “The cost of motoring is a burden on motorists and people are looking to save money by driving less or deferring services.”
Halfords said its Autocentres car mechanic business saw like-for-like sales growth of 2.7 per cent with strong demand for servicing and tyres. But it said first half operating profits at the business, which it bought from Nationwide Autocentres last year, will be lower than in 2010.
There has been a five per cent drop in petrol sales volumes in the UK in the half year, according to AA figures out this week.
But shares in Halfords closed more than eight per cent up yesterday.