Porsche values climbed by 0.4% in October according to the HAGI P Index, while the overall market begins to normalise following record sales.
The HAGI Top Index - the overall measure for "exceptional historic automobiles" - rose by 9.3% in August, boosted by the Pebble Beach auctions and the record sale of Juan Manuel Fangio's Mercedes W196 in July.
However, in October the index is down 1.86%. Yet remove Ferrari from the equation and the Top Index has still risen by 3.2%.
This is due to Ferrari excelling in September as a 1953 250 GTO became the world's most valuable car ever sold at $52m, sending values rocketing to unsustainable heights.
"As a cautious note, we do not think that the growth rates we are seeing this year are sustainable long-term," warned HAGI on release of August's results.
The results for the year as a whole are positive, with the Top Index having seen a 29.0% increase for the year to date. Ferrari is up 32.6%, while Porsche has seen values rise by 19.5% in the past 12 months.
The positive growth shown by Porsche comes ahead of major sales of the marque's top models in 2014, including competition models from the Harburg Collection and some stunning examples at RM Auctions' Arizona sale.
In August, a 1955 Porsche 550/1500 RS Spyder sold for $3.7m through Mecum Auctions.
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