What FMCSA actually requires (the short version)
If your truck or trailer runs interstate at over 10,001 lbs GVWR, hauls hazmat, or carries paying passengers, FMCSA 49 CFR 390.21 requires your legal business name and USDOT number on both sides of the power unit. Letters must be at least 2 inches tall, in a color that sharply contrasts the background, and readable from 50 feet in daylight. MC numbers, KYU permits, and intrastate-only DOTs follow the same sizing rules in most states.
Magnets and chalk-marker work for a weekend, not for a Level 1 inspection. North Carolina state troopers and Guilford County weigh-station officers routinely cite removable lettering, faded numbers, and the wrong contrast (yellow on white, dark gray on black) as Form-A violations. A $25 magnet can turn into a $500 fine plus an out-of-service order that pulls the truck off the road until corrected.