Average U.S. per-vehicle incentives from auto makers remained stable in December from the prior month as many auto makers started their end-of-year promotions early, according to car-shopping website Edmunds.com.

Edmunds.com reported the average new-car incentive was $2,286 per vehicle in December, a 0.8% decrease from the prior month and a 1% decline from a year earlier.

Edmunds said the results suggest most end-of-year promotions were available as early as November, which gave car buyers more time to consider all of the deals available. The firm noted deals started to increase in November, when auto makers raised incentives nearly 6%.

Ford Motor Co. (F) reported the biggest month-over-month increase in incentives among the top six auto makers for December, up 15%. General Motors Co.'s (GM) incentive spending rose 5.2% sequentially while Toyota Motor Corp. (TM, 7203.TO) reported a 3.7% increase.

Meanwhile, Japanese auto maker Nissan Motor Co. (NSANY, 7201.TO) posted the largest month-to-month decline in incentive spending, with a 14% decrease. Honda Motor Co. (HMC, 7267.TO) posted a 6.3% month-to-month decline. Incentive spending at Chrysler Group LLC slipped 3.4%.

For 2012, Edmunds said the industry-wide average true cost of incentives totaled $2,206 a vehicle, a 5.1% decline from 2011. Only Nissan increased its incentive spending year-over-year, up 7.4% for 2012. However, Edmunds said the increased incentive spending didn't help Nissan's market share, which is expected to fall slightly in 2012 from a year earlier.

"It's likely that incentives will continue a slow but steady rise in the coming months," said Senior Analyst Jessica Caldwell. "There will be bigger discounts available on 2012 models through the summer to help dealers clear the way for incoming 2013 models."

Ms. Caldwell noted 2013 models already account for about 6.5% of new-car sales.

Edmunds.com's monthly incentives report takes into account all auto makers" various U.S. incentives programs and bases its calculations on sales volume as well as the proportion of vehicles for which each type of incentive was used.

Write to Nathalie Tadena at nathalie.tadena@dowjones.com

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