In plain English
FAR 52.251-2, Interagency Fleet Management System Vehicles and Related Services, is a contract clause prescribed at FAR 51.205, most recently dated Jan 1991. The complete official text is reproduced below, verbatim, from GSA's published FAR source files.
Its text contains no sentence requiring insertion into subcontracts; see the flowdown section below for what that does and does not mean.
Does it flow down to subcontracts?
No flowdown mandate found in the clause text
We scanned the full clause text and found no sentence directing the contractor to insert this clause into subcontracts. That is a statement about the text, not legal advice: a prime contractor may still flow terms down contractually, an agency supplement (DFARS, VAAR, …) may add requirements, and clauses listed below (if any) may order this clause into subcontracts from the outside.
Where it's prescribed
As prescribed in 51.205 , insert the following clause:
Prescribing reference: FAR 51.205.
The official text, verbatim
As prescribed in 51.205 , insert the following clause:
Interagency Fleet Management System Vehicles and Related Services (Jan 1991)
The Contracting Officer may issue the Contractor an authorization to obtain interagency fleet management system (IFMS) vehicles and related services for use in the performance of this contract. The use, service, and maintenance of interagency fleet management system vehicles and the use of related services by the Contractor shall be in accordance with 41 CFR 101-39 and 41 CFR 101-38.301-1.
(End of clause)