The SMMT said on Thursday there were a total of 170,866 new registrations last month, but it expected a recovery in November as there were two less reporting days this October compared with last year.
"Despite a dip in October, the new car market is still running parallel with last year's record total," said Christopher Macgowan, SMMT chief executive.
"We expect a slight upturn in November figures which will leave the increasingly strong December market as the decider," he said.
The SMMT said new registration plates for private buyers fell 13.3% from a year earlier to 75,372, compared with a fall of 6% in the previous month.
The SMMT said it has just revised its forecast for 2005 new sales down to 2.465 million from the 2.48 million it had projected in July because of an expected slowdown in the economy.
The Ford Focus returned to the top of the best sellers' list in October, outpacing Renault's Mégane by only 144 units after Mégane volumes soared 76%. Lower medium sector cars took the first four places in October as the segment grew by 6.7&.
Last month's top seller, the Vauxhall Corsa slipped to sixth spot in October. Sports cars, MPVs and dual purpose 4x4s continued to show good gains, with volumes up 11.9, 3.8 and 8.4% respectively in October.
Diesel penetration reached the highest ever monthly total with 37.2% market share in October, over four percentage points higher than in any other month and up on last year's 29.7%.