UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of report (Date of earliest event reported): May 7, 2025

ALLISON TRANSMISSION HOLDINGS, INC.

(Exact Name of Registrant as Specified in Charter)

Registrant’s telephone number, including area code: (317) 242-5000

Not Applicable

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instructions A.2. below):

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§ 230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§ 240.12b-2 of this chapter).

Emerging growth company ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

As further described below in Item 5.07 of this Current Report on Form 8-K, on May 7, 2025, at the annual meeting of stockholders (the “annual meeting”) of Allison Transmission Holdings, Inc. (the “Company”), the Company’s stockholders approved an amendment (the “Exculpation Amendment”) to the Company’s Second Amended and Restated Certificate of Incorporation to allow for exculpation of the Company’s officers from liability in specific circumstances. On May 8, 2025, the Company filed with the Secretary of State of the State of Delaware a Certificate of Amendment to the Company’s Second Amended and Restated Certificate of Incorporation that sets forth the Exculpation Amendment (the “Certificate of Amendment”).

A description of the Exculpation Amendment is set forth in the Company’s definitive proxy statement filed with the Securities and Exchange Commission on March 26, 2025 in the section entitled “Proposal No. 3—Approval of the amendment to our Second Amended and Restated Certificate of Incorporation to provide for exculpation of officers.” The foregoing description of the Exculpation Amendment is qualified in its entirety by reference to the full text of the Certificate of Amendment, a copy of which is filed as Exhibit 3.1 to this Current Report on Form 8-K and incorporated herein by reference.

On May 7, 2025, the Company held its annual meeting. At the annual meeting, stockholders took the following actions:

The vote tabulation for each proposal follows:

Proposal 1 – Election of Directors.

Proposal 2 – Ratification of Appointment of PwC.

Proposal 3 – Approval of the Exculpation Amendment.

Proposal 4 – Advisory Vote on Executive Compensation.

Proposal 5 – Advisory Vote on the Frequency of Future Advisory Votes on Executive Compensation.

In accordance with the Board of Directors’ recommendation and the voting results on this advisory proposal, the Board of Directors has determined that the Company will hold future advisory votes on Executive Compensation every year until the next required advisory vote on the frequency of advisory votes on Executive Compensation, which will occur no later than the Company’s annual meeting of stockholders in 2031.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.