SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

Fuel Systems Solutions, Inc.

(Name of Issuer)

Common Stock

(Title of Class of Securities)

35952W103

(CUSIP Number)

Pier Antonio Costamagna

Via Ospedale, 35 12062 Cherasco CN ITALY

+393356501316 or +393356185546

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

March 31, 2016

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-l(g), check the following box ¨.

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See §240.13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

This Schedule 13D relates to the common stock, par value $0.001 per share (the “Common Stock”) of Fuel Systems Solutions, Inc., a Delaware corporation (the “Issuer”). The principal executive offices of the Issuer are located at 780 Third Avenue, 25th Floor, New York, NY 10017.

(a) – This Schedule 13D is being filed by Pier Antonio Costamagna and Carla Borgogno (the “Filing Persons”). Pier Antonio Costamagna owns shares in his individual capacity. Carla Borgogno is the spouse of Pier Antonio Costamagna. In the aggregate, the Filing Persons own 8.7% of the Common Stock. Each of the Filing Persons disclaims beneficial ownership of the shares owned by the other Filing Person.

(b), (c) and (f) – Each of the Filing Persons is an Italian citizen, with a business address of Via Ospedale, 35 12062 Cherasco CN ITALY. Pier Antonio Costamagna retired as an executive officer of the Issuer and as the General Manager of MTM, S.r.L., a wholly owned subsidiary of the Issuer, effective February 5, 2014.

(d) and (e) – During the last five years, neither of the Filing Persons has been convicted in a criminal proceeding, and no such person was a party to a civil proceeding of a judicial or administrative body of competent jurisdiction as a result of which he or she was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws in the United States or finding any violation with respect to such laws.

Not applicable. See Item 4 below.

Each of the Filing Persons holds the shares for investment purposes.

On September 1, 2015, the Issuer, Westport Innovations Inc., an Alberta, Canada corporation (“Westport”) and Whitehorse Merger Sub Inc., a Delaware corporation and a direct wholly owned subsidiary of Westport (“Merger Sub”) entered into an Agreement and Plan of Merger (the “Merger Agreement”) pursuant to which, the Issuer will be merged with and into Merger Sub (the “Merger”), with the Issuer surviving the Merger.

On March 6, 2016, the Issuer, Westport and Merger Sub entered into Amendment No. 1 to the Merger Agreement (the “Merger Agreement Amendment”), which, among other things, amended the Merger Agreement to provide for a collar mechanism as opposed to a fixed number for calculating the exchange ratio. Pursuant to the Merger Agreement Amendment, the exchange ratio varies based upon the NASDAQ volume weighted average price of Westport common shares during a specified measuring period. Also on March 6, 2016, Westport and Cartesian Capital Group (“Cartesian”) entered into an amendment (the “Cartesian Amendment”) to their previously announced financing agreement between Westport and an affiliate of Cartesian which, among other things, (i) amended the financing agreement to provide that the second tranche of financing to be provided by an affiliate of Cartesian, an investment of $17.5 million in cash in exchange for a Westport convertible note, will close simultaneously with the closing of the Merger and (ii) amended the convertible note valuation price to reflect the changes to the calculation of the exchange ratio set forth in the Merger Agreement Amendment.

As more fully explained in the press release issued by the Reporting Persons on March 31, 2016, which is attached hereto as Exhibit 99.1 and incorporated herein by reference, the Reporting Persons strongly believe that the Merger Agreement Amendment materially alters the terms and structure of the transaction contemplated by the original Merger Agreement in a manner that is not in the best interests of the Issuer or its stockholders.

Accordingly, the Reporting Persons believe there is a need, and they intend, to, vote AGAINST the proposal to adopt the Merger Agreement (as amended) and approve the Merger between the Issuer and Merger Sub at the special meeting of stockholders to vote.

The Reporting Persons intend to review their investments in the Issuer on a continuing basis. Depending on various factors (including, without limitation, the Issuer’s financial position and strategic direction, actions taken by the Board, price levels of the shares of the Common Stock, other investment opportunities available to the Reporting Persons, concentration of positions in the portfolios managed by the Reporting Persons, market conditions and/or general economic and industry conditions), the Reporting Persons may take such actions with respect to their investments in the Issuer as they deem appropriate (including, without limitation, purchasing additional shares of the Common Stock or other financial instruments related to the Issuer or selling some or all of their beneficial or economic holdings, engaging in hedging or similar transactions with respect to the securities relating to the Issuer and/or otherwise changing their intention with respect to any and all matters referred to in Item 4 of Schedule 13D).

(a) – Pier Antonio Costamagna is the direct beneficial owner of 1,582,043 shares, or approximately 8.7% of the Outstanding Shares.

Carla Borgogno is the indirect beneficial owner of 1,582,043 shares, or approximately 8.7% of the Outstanding Shares, which are owned directly by Pier Antonio Costamagna.

The foregoing beneficial ownership percentage is based upon 18,094,043 shares of Common Stock issued and outstanding as of March 7, 2016, as reported by the Issuer in its Annual Report on Form 10-K for the year ended December 31, 2015 (File No. 001-32999), filed with the Securities and Exchange Commission on March 14, 2016.

(b) – Pier Antonio Costamagna has the sole power to vote the 1,582,043 shares that he holds directly and shares with his wife, Carla Borgogno, the power to dispose of the 1,582,043 shares that he holds directly.

(c) – The transactions in the class of securities reported on that were effected during the past sixty days by the Reporting Persons are described below. The transactions were effected by MPS Capital Service - Banca Monte dei Paschi di Siena, Via Cavour 1 12051 Alba (CN) ITALY as the investment adviser on behalf of the Reporting Persons.

(d) – Not applicable.

(e) – Not applicable.

Each of the Filing Persons has entered into the Joint Filing Agreement attached hereto as Exhibit 99.2 and incorporated herein by reference.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete, and correct.

DATED: April 4, 2016

Exhibit 99.1

Fuel Systems Solutions Co-Founder Intends to Vote AGAINST Proposed Merger with Westport

Beneficial Owner of Approximately 8.8% of Outstanding FSS Shares Sends Letter to FSS Board of Directors

Believes Cartesian Agreement Represented a Clear Breach of the Terms of the Merger Agreement

Cartesian Agreement Materially and Irrevocably Changes Future of Westport

CHERASCO, Italy – March 31, 2016 – Pier Antonio Costamagna, a co-founder of Fuel Systems Solutions, Inc. (“FSS”) (NASDAQ: FSYS) today sent a letter to the board of directors of FSS regarding his intention to vote against the proposed merger of Westport Innovations, Inc. (“Westport”) (TSX:WPT / NASDAQ:WPRT) and FSS. Mr. Costamagna has sole voting power over 1,584,589 shares of FSS common stock, representing approximately 8.8% of outstanding shares.

A copy of the letter follows:

Fuel Systems Solutions, Inc.

780 Third Avenue, 25th Floor

New York, New York 10017

Members of the Board:

I write to you out of serious concern for the future of my significant investment in Fuel Systems Solutions, Inc. (“FSS”). As you are aware, I am a co-founder of FSS and have sole voting power over 1,584,589 shares of FSS common stock, representing approximately 8.8% percent of the outstanding shares of FSS common stock as of March 7, 2016, making me one of FSS’ largest stockholders.

I have become increasingly concerned about recent developments involving FSS since the announcement of the merger agreement between FSS and Westport Innovations, Inc. (“Westport”) on September 1, 2016. As a result, I intend to vote AGAINST the proposal to adopt the merger agreement and approve the merger between FSS and Westport at the special meeting.

Most of all, I am concerned by how significantly Westport and this transaction have changed since this deal was first announced. My reasons include:

With the uncertainty of Cartesian’s ongoing role, it is surprising FSS has decided to press forward with the amendments to the merger agreement. This is particularly perplexing when the Board was also seriously discussing an all cash offer with a still undisclosed third party as of February 1, 2016 but determined to reject it for this amended offer.

My concern over the Cartesian investment agreement is not only on the debt incurred but the rights gained by Cartesian that alter the future of Westport. Specifically, the $71 million investment gives Cartesian:

In my view these are significant provisions that materially and irrevocably change Westport and therefore have significant impacts on my future investment as an FSS stockholder.

Following the disclosure of Westport’s fourth quarter and full year financial results yesterday, I felt compelled to make my view known publicly. I am concerned that I am being asked to take equity in a now highly indebted company that has seen its cash position continue to rapidly deteriorate. Westport’s cash balance decreased 70.4% year over year. Furthermore, the negative change in consolidated adjusted EBITDA quarter to quarter shows me a continued and concerning inability to successfully generate earnings. Put simply, Westport’s fourth quarter and full year financial results announced this week further demonstrate its incredibly significant challenges ahead. The future within Westport is simply too uncertain for me and I cannot get comfortable with accepting equity in this company.

It is this troubling cash position that contributes to my belief that the significant amount of the Cartesian indebtedness that was incurred by Westport could negatively affect the operation of the envisaged combined company. To be specific, I believe that the terms of the Cartesian investment agreement could significantly restrict the combined company going forward by imposing certain negative covenants, including borrowing restrictions. These negative covenants and other restrictions provide significant “control” over the combined company’s operations without any apparent benefit to FSS stockholders.

In light of these alarming events and my concern about the future of FSS following a combination with Westport, I intend to vote AGAINST the proposal to adopt the merger agreement and approve the merger at the special meeting.

Thank you.

Sincerely,

Media Contacts:

Abernathy MacGregor

Pat Tucker or Cia Williams

212.371.5999

pct@abmac.com / cew@abmac.com

Exhibit 99.2

JOINT FILING AGREEMENT

WHEREAS, the statement or amended statement on Schedule 13D to which this agreement is an exhibit (the “Joint Statement”) is being filed on behalf of the undersigned persons (collectively the “Filing Persons”); and

WHEREAS, the Filing Persons prefer to file the Joint Statement on behalf of all of Filing Persons rather than individual statements on Schedule 13D on behalf of each of the Filing Persons;

NOW, THEREFORE, the undersigned hereby agree as follows with each of the other Filing Persons:

Dated: April 2, 2016