Classic Porsches outperformed their Ferrari and Mercedes-Benz counterparts in the first 11 months of 2016.

That's according to Historic Auto Group's (HAGI) classic car indices.

A 1956 Porsche 550 Spyder sold for $6m this year

The German manufacturer's classic machines have grown in value by 12.3% so far this year, following a bumper November that saw price growth reach 6.5%.

That compares with 8.5% for Mercedes-Benz vehicles so far in 2016, and 3.3% for Ferrari.

This year we've seen a 1956 Porsche RS Spyder sell for $6m - a record for its type, while a 1995 Porsche 911 GT2 achieved $2.4m.

The HAGI Top index, which tracks the performance of the world's leading classics across a variety of marques, has posted 8.1% growth in the past 11 months.

With just one month to go, it appears the index will be down on the 17.6% gains seen last year, and the 25.1% rise achieved in 2014.

Yet 8.1% growth is far beyond most traditional investments.

Dietrich Hatlapa, HAGI's founder, told Classic Driver recently that an increase in vehicles appearing for sale is behind the drop-off.

"Demand hasn't tailed off, but there has been a notable increase in supply," he said.

"There are more dealers, more auction houses, and more cars consigned to an increasing number of sales."

It will be fascinating to see whether the slowdown continues in 2017. A good early indication will be the Scottsdale, Arizona auctions in January, where an $8m-valued 1963 Jaguar E-type lightweight competition racer will be one of the highlights at Bonhams.