49 CFR §387.305
Verified against eCFR.gov as of June 20, 2026View official text on eCFR.gov ↗
The following combinations will be regarded as one motor vehicle for purposes of this part, (a) a tractor and trailer or semitrailer when the tractor is engaged solely in drawing the trailer or semitrailer, and (b) a truck and trailer when both together bear a single load.