Both companies are owned by the Agnelli family holding company Exor SpA <EXOR.MI>, which said on Thursday the shareholder simplification would give more clarity and efficiency and that it intended to maintain its 30 percent stakes.
In trading on Friday, Fiat savings shares <FIAn.MI> were up 16 percent and its preference shares <FIA_p.MI> were up 19 percent. Fiat Industrial's savings share s<FIn.MI> were up 32.5 percent and the preference were up 37 percent.
A Milan trader said the prices were moving in line with the conversion rates for Fiat and Fiat Industrial.
Analysts said the share conversions will save dividend payments for the two companies. Mediobanca said in a note it would ease any Fiat Industrial disposal of truck unit Iveco by avoiding a savings shareholder vote.
The ordinary shares in both companies were not impacted by the conversion factor, the trader said.
Instead, Fiat ordinaries were down 5 percent on its results statement. Fiat Industrial fell 1.9 percent after opening up.
On Fiat, analysts said accounting changes at its U.S. unit Chrysler boosted earnings, while net debt came in higher than expected. Credit Suisse said in a note parts of the 2011 guidance "appear very difficult to achieve".
On Fiat Industrial, Exane upped its price target by 9 percent. The Milan trader said his firm had upgraded the stock.
Fiat Industrial reversed initial rises and was suspended down 1.9 percent. A second trader said hedge funds were arbitraging the ordinary share with the preference shares, adding the results were "good, surprisingly good".
(Reporting by Michel Rose and Nigel Tutt; Editing by David Holmes)