"In addition to Bridge Formula weight limits, Federal law states that single axles are limited to 20,000 pounds, and axles spaced more than 40 inches and not more than 96 inches apart (tandem axles) are limited to 34,000 pounds. Gross vehicle weight is limited to 80,000 pounds (23 U.S.C. 127)."
Bridge Formula Weights, FHWA-HOP-19-028, p. 2 — FHWA, August 2019. Public domain.
Note that a tandem is defined by a spacing range, not just a maximum: axles closer together than 40 inches are not a tandem under federal law. Note also the statutory carve-out that makes the standard 80,000 lb van legal at all — two consecutive sets of tandem axles may carry 34,000 lb each, 68,000 lb between them, provided the outer axles of the two tandems are at least 36 feet apart. The bare bridge formula at that geometry allows only 66,000 lb.
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