1. One side only
The rule says both sides of the power unit. Trucks lettered only on the driver's side — or with one decal lost to a pressure wash — are the most common write-up. Decals are sold in sets of two for a reason.
2. No contrast
Black number on a dark blue door technically exists; legally it doesn't. The standard is letters that "contrast sharply in color with the background." White on dark paint, black on white or chrome — pick the one an inspector can't argue with.
3. Too small for 50 feet
There's no federal letter-height number — the test is legibility from 50 feet in daylight while the truck is parked. In practice, 2-inch letters pass comfortably; the 1-inch vinyl some shops sell to fit a fuel door does not.
4. Somebody else's number
Bought a used truck with the old owner's USDOT still on the door? Operating under a number that isn't yours is a serious problem, and several states explicitly require removing markings before resale. Strip the old vinyl the day the title transfers.
5. Missing the state extras
Federal-perfect marking can still fail a state check: a missing MT/MO/OK/WY suffix, a state carrier number, or New Jersey's 3-inch GVWR display. The free compliance check catches these for your state in 60 seconds.
The 5-minute insurance policy
A correct set of decals costs less than the time a roadside write-up burns. Cut to spec, high contrast, both sides, applied in minutes — and if you mess up the install, we replace it free.
General information, not legal advice. Rules change — confirm current federal and state requirements with the FMCSA and your state agency before you operate. Our state summaries were verified against primary sources July 2026.
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