UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM SD

Specialized Disclosure Report

ALLISON TRANSMISSION HOLDINGS, INC.

(Exact name of registrant as specified in its charter)

Eric C. Scroggins

Vice President — General Counsel and Secretary

Allison Transmission Holdings, Inc.

One Allison Way

Indianapolis, IN 46222

(317) 242-5000

(Name and telephone number, including area code, of the person to contact in connection with this report.)

Check the appropriate box to indicate the rule pursuant to which this form is being filed, and provide the period to which the information in this form applies:

Section 1 – Conflict Minerals Disclosure

Item 1.01 Conflict Minerals Disclosure and Report

Conflict Minerals Disclosure

Allison Transmission Holdings, Inc. (the “Company”) performed a reasonable country of origin inquiry (“RCOI”), of its global supply chain of direct ferrous, metallic, and electrical component suppliers regarding whether its necessary gold, tantalum, tin and tungsten (“conflict minerals”) originated from the Democratic Republic of the Congo or an adjoining country (the “Covered Countries”). Most of the responses the Company received indicated that the conflict minerals in the suppliers’ components and materials either did not originate from a Covered Country, were not necessary to the functionality of the components and materials or the source country was undeterminable. Further information regarding the Company’s RCOI is included in the Company’s Conflict Minerals Report, which is attached hereto as Exhibit 1.01.

During its RCOI, the Company was not able to determine whether its necessary conflict minerals may have originated from the Covered Countries and may not have come from recycled or scrap sources. As such, the Company proceeded to exercise due diligence on the source and chain of custody of the conflict minerals. The Conflict Minerals Report includes a discussion of the due diligence procedures performed and the disclosures required by the Securities and Exchange Commission.

The Company has disclosed a copy of its Conflict Minerals Report on its website at www.allisontransmission.com.

Item 1.02 Exhibit

The Company’s Conflict Minerals Report is filed herewith as Exhibit 1.01.

Section 2 – Exhibits

Item 2.01 Exhibits

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 duly authorized undersigned.

Exhibit 1.01

Allison Transmission Holdings, Inc.

Conflict Minerals Report

For the Year Ended December 31, 2018

This Conflict Minerals Report for the year ended December 31, 2018 is presented to comply with Rule 13p-1 (“Rule 13p-1”) under the Securities Exchange Act of 1934, as amended. Rule 13p-1 was adopted by the Securities and Exchange Commission (“SEC”) to implement reporting and disclosure requirements related to conflict minerals as directed by the Dodd-Frank Wall Street Reform and Consumer Protection Act of 2010 and imposes certain reporting obligations on SEC registrants who manufacture or contract to manufacture products that contain conflict minerals which are necessary to the functionality or production of their products. “Conflict minerals” are defined as tantalum, tin, tungsten and gold. These requirements apply to registrants whatever the geographic origin of the conflict minerals and whether or not they fund armed conflict in the Democratic Republic of Congo (“DRC”) or an adjoining country (together with the DRC, the “Covered Countries”). This Conflict Minerals Report has not been subject to an independent private sector audit.

1. Company Overview and Product Description

Allison Transmission Holdings, Inc. and its subsidiaries (“Allison”, “we,” “us” or “our”) design and manufacture fully-automatic transmissions for medium- and heavy-duty commercial vehicles, medium- and heavy-tactical U.S. defense vehicles and hybrid-propulsion systems for transit buses. We generate our net sales primarily from the sale of transmissions, transmission parts, support equipment, defense kits, engineering services and extended transmission coverage to a wide array of original equipment manufacturers, distributors and the U.S. government.

We conducted an analysis of our products and found that conflict minerals are necessary to the production or functionality of our commercial, defense and hybrid products.

A complete description of our products can be found in our Annual Report on Form 10-K for the year ended December 31, 2018, filed with the Securities and Exchange Commission on February 26, 2019.

2. Reasonable Country of Origin Inquiry

We conducted a good faith reasonable country of origin inquiry (“RCOI”) to determine whether the necessary conflict minerals originated in the Covered Countries. As a company engaged in the design and manufacture of commercial and defense fully-automatic transmissions and hybrid-propulsion systems for transit buses, we are several levels removed from the actual mining of conflict minerals. We do not make purchases of raw ore or unrefined conflict minerals and we conduct no purchasing activities in the Covered Countries. In order to determine whether conflict minerals used in the manufacture of our products originated in the Covered Countries, we relied on our direct suppliers to provide information on the origin of the conflict minerals contained in components and materials supplied to us, including sources of conflict minerals that are supplied to them from their suppliers.

Scope of RCOI. Our RCOI included conducting an inquiry of our global supply chain of direct ferrous, metallic, and electrical component suppliers, using the Responsible Minerals Initiative’s (“RMI”) Conflict Minerals Reporting Template (“CMRT”) to identify the smelters and refiners responsible for processing the conflict minerals.

To assist in the collection of data and in engaging our suppliers, we enlisted a third party data collection and analysis vendor to conduct outreach, training and verification of supplier responses. As part of our global supply-chain survey, we received representations from our suppliers regarding the source of conflict minerals and compared the smelters and refiners identified in the CMRT responses against the list of smelter and refinery facilities which have been identified as conflict free by programs such as RMI’s Responsible Minerals Assurance Process for conflict minerals. In an effort to obtain the highest practical response rate, our process included multiple rounds of communication and follow-up through mail, email and telephone calls. We created and maintained a database of this information.

RCOI Results. Of our direct ferrous, metallic, and electrical component suppliers, 100% were within the scope of our RCOI. We received responses from approximately 61% of the suppliers. We reviewed the responses against criteria developed to determine which responses required additional engagement, including incomplete responses and inconsistencies within the data reported. We worked with these suppliers to obtain a revised response or additional clarity regarding their submission.

3. Due Diligence

We conducted due diligence on the source and chain of custody of the conflict minerals that are included in our products and for which, based on our RCOI, we have not been able to determine whether the conflict minerals may have originated in the Covered Countries and may not have come from recycled or scrap sources.

Design of Due Diligence. Allison’s due diligence procedures were designed to conform with the Organisation of Economic Co-Operation and Development (“OECD”) Due Diligence Guidance for Responsible Supply Chains of Minerals from Conflict-Affected and High-Risk Areas, which is an internationally recognized due diligence framework. Our due diligence process included:

Establishing Company Management Systems. We adopted a conflict minerals policy that governs the supply-chain of conflict minerals from the Covered Countries. In addition, we organized an internal management team to support our supply chain due diligence efforts. This cross-functional team, led by our Chief Compliance Officer and a team of subject matter experts from relevant functional areas, oversees our conflict minerals due diligence process. Senior management and the Audit Committee of our Board of Directors are briefed about the results of our due diligence efforts.

Identifying and Assessing Risk in the Supply Chain. Through our RCOI process, we solicited all ferrous, metallic, and electrical component direct suppliers with a CMRT to obtain information about the source of any conflict minerals. Our efforts largely focused on confirming the accuracy and quality of the representations our direct suppliers made regarding the source and chain of custody of their conflict minerals and comparing the smelters and refiners identified in the CMRT responses against the list of smelter and refinery facilities which have been identified as “conflict free”.

Designing and Implementing a Strategy to Respond to Risks. Our conflict minerals team is responsible for identifying and responding to risks in our supply chain. In addition, we monitor and track information received from suppliers, including a list of smelters and refiners identified through CMRT responses.

Supporting Independent Third-Party Audits of Supply Chain. We do not have a direct relationship with the conflict minerals smelters and refiners; therefore, we rely on the RMI’s Responsible Minerals Assurance Process to manage and report results of the independent third-party audits and the related certifications.

Reporting on Supply Chain Due Diligence. This conflict minerals report is being filed with the SEC as an exhibit to our specialized disclosure report on Form SD and is available on our website at www.allisontransmission.com.

4. Due Diligence Results.

Because we have not received sufficient and complete responses from all suppliers and many of the supplier responses provided data at a company level rather than at a product level, we do not have sufficient information to conclusively determine the countries of origin or chain of custody of conflict minerals in our products. Based on the CMRT responses from our direct suppliers, we believe that, to the best of our knowledge, the smelters and refiners listed in Annex A to this conflict minerals report may have been used to process the conflict minerals contained in our products.

5. Future Due Diligence Measures.

As we further develop our due diligence program in calendar year 2019, we intend to continue implementing the following steps to improve the information gathered from our due diligence efforts and to further mitigate the risk that our necessary conflict minerals do not benefit armed groups:

ANNEX A