Could be wrong, but my understanding is that maximum legal front axle weight is 20,000 lbs. With dual axle rears at 34,000, maximum theorectical gross weight for a 3 axle straight truck can be 54,000 lbs. There are other limitations to observe such as tire capacity, structural limits, etc. Add pony axles, and the maximum gross weight can increase. Once you exceed 55,000 lb registered gross weight, you start getting into the Federal Heavy Highway Use Tax with its associated costs and filings.