The official Cost of Acquisition (COA) ratio for the Tata Motors demerger is 68.85% for Tata Motors Passenger Vehicles (TMPV) and 31.15% for Tata Motors (Commercial Vehicles). 

This ratio is used to apportion your original purchase cost across the shares of the two newly formed entities for tax purposes.

Demerger Details

• Share Entitlement Ratio: Shareholders received one equity share of the new commercial vehicle entity (now renamed to Tata Motors) for every one share held in the original Tata Motors entity (now renamed Tata Motors Passenger Vehicles Ltd) as of the record date, October 14, 2025.
• Cost Apportionment: Your total original cost of acquisition for the shares is split based on the official 68.85 % Tata Motors Passenger Vehicles (TMPV) & 31.15% Tata Motors Commercial Vehicles(TMCV) Ratio. This ensures your overall investment value remains the same, just distributed across two stocks.
• Tax Implications: The demerger itself does not attract any immediate tax. Tax is applicable only when you sell the shares. The holding period for both new stocks starts from your original purchase date of the un-demerged Tata Motors shares

Example of COA Calculation

• If you purchased 100 shares of Tata Motors at ₹800 per share (total cost of ₹80,000) before the demerger:

Entity	New Shares Held	Apportionment Ratio	Adjusted Cost per Share	Total Adjusted Cost
Tata Motors Passenger Vehicles Ltd (TMPV)	100	68.85%	₹550.80 (68.85% of ₹800)	₹55,080
Tata Motors (Commercial Vehicles)	100	31.15%	₹249.20 (31.15% of ₹800)	₹24,920

The combined adjusted cost remains ₹800 per share (₹550.80 + ₹249.20), and the total cost of your holdings remains ₹80,000. Your broker or depository should automatically adjust the cost of acquisition in your portfolio, or you can use these figures for your records. 

Source: https://www.bseindia.com/xml-data/corpfiling/AttachLive/bece4d69-3276-4305-88a5-440ed2a2660e.pdf