Three federal numbers cover almost everything a CDL knowledge test asks about weight: 80,000 pounds gross, 20,000 pounds on a single axle, and 34,000 pounds on a tandem. They come from 23 U.S.C. 127 and 23 CFR 658.17, and they govern the Interstate System and reasonable access to it.
All three are scale readings, not sticker ratings. That is what separates this from the Class A versus Class B question, which turns on what a manufacturer stamped on a plate. Here it is what the vehicle weighs today, loaded.
The three numbers
- 80,000 pounds gross. The maximum gross vehicle weight on the Interstate System, except where the bridge formula dictates a lower figure.
- 20,000 pounds on any one axle. That includes any single axle inside a group of axles. Federal law defines a single axle by spacing: axle centers not more than 40 inches apart.
- 34,000 pounds on a tandem. A tandem is consecutive axles whose centers are more than 40 inches and not more than 96 inches apart.
All three apply at once. A truck can cross the scale at 78,000 pounds gross, comfortably under the federal ceiling, and still be written up because one tandem carries 35,400. Gross weight is a limit, not a permission.
What the bridge formula is actually doing
The bridge formula is about the bridge, not the truck. The same load spread over a longer wheelbase presses less on any single span; bunch the axles together and you concentrate it. So the rule asks how far apart your axles sit before deciding how much they may carry.
The formula in 23 CFR 658.17(e) is:
W = 500 × ( LN ÷ (N − 1) + 12N + 36 )
- W is the maximum weight allowed on that group of axles.
- L is the distance in feet between the outer axles of the group.
- N is the number of axles in the group.
Nobody runs that arithmetic at a scale house; FHWA publishes a bridge table to read off. What the CDL manual asks you to understand is the principle: a bridge formula permits less weight on axles that sit closer together, so bridges and roadways are not overloaded.
The part that catches people out is that it applies to every group of two or more consecutive axles, not just the vehicle end to end. FHWA's worked example is a single-unit truck at 57,000 pounds gross: legal on gross, on every single axle, on its tandem and across axles 1 through 4, yet over the limit across axles 2 through 4, because those three sit only 9 feet apart. The remedies are always the same three: reduce the load, add an axle, increase the spacing.
The 36-foot exception, and why rigs look alike
Run the formula on a five-axle tractor-semitrailer and the drive tandem plus the trailer tandem come out low. On tandems spaced 36 to 38 feet apart, the formula alone would allow only 66,000 to 67,500 pounds across those four axles.
Federal law carves out one exception for exactly this case. Two consecutive sets of tandem axles may carry 34,000 pounds each, 68,000 across the four, provided the overall distance between the first and last axle of those tandems is 36 feet or more. On a five-axle rig that means axles 2 and 5 spaced at least 36 feet apart.
Put a steer axle of about 12,000 pounds in front of 34,000 and 34,000 and you land on 80,000. That is not a coincidence: the familiar 12,000 / 34,000 / 34,000 split is the shape this exception rewards.
Where the weight sits, and how you move it
Federally the steer axle is just a single axle, so 20,000 pounds is its ceiling, but two other limits usually bite first. A vehicle may not run tires carrying more weight than the figure marked on the sidewall (49 CFR 393.75(f)), and steer tires and suspensions commonly top out near 12,000 to 13,000 pounds. Some states set a steer axle limit of their own.
Balance matters for control as much as legality, and the manual is blunt about the failure modes. Too much weight on the steering axle makes the truck hard to steer and can damage the axle and tires. Too little on it, from shifting the load too far back, leaves the steering too light to be safe. Too little on the drive axles costs traction, and the wheels spin easily in bad weather. Cargo stacked high raises the center of gravity, which turns a curve or a swerve into a rollover, so heavy pieces belong under light ones.
When the gross is legal but one group is heavy, you move the load rather than shed it. Two things slide:
- The trailer tandems. Slide them back and weight transfers forward onto the drives. Slide them forward and weight transfers rearward onto the trailer axles. One hole moves a few hundred pounds, but the amount depends on the trailer's hole spacing and how the freight sits, so treat any per-hole number as a rule of thumb and re-weigh.
- The sliding fifth wheel. Move it forward and weight shifts from the drives onto the steer axle; move it back and the steer axle gets lighter. Then lock it, and check that it is locked, before you pull.
Two cautions. Sliding the tandems forward shortens the spacing across the group, which lowers what the bridge formula allows, so you can cure an axle and create a bridge violation in one move. It also changes your kingpin-to-rearmost-axle distance, which some states regulate: California caps it at 40 feet for a semitrailer with two or more axles, 38 feet with one axle.
Where federal limits stop and state law starts
- The 80,000-pound figure is an Interstate figure. 23 CFR 658.17 applies to the Interstate System and reasonable access to it. Elsewhere states set their own limits, and some are far higher; Michigan permits well above 80,000 pounds on its own roads for rigs with enough axles and spacing.
- Grandfather rights survive. Weights legally authorized under state law on July 1, 1956, and axle configurations grandfathered under state groups-of-axles tables or formulas on January 4, 1975, remain exempt.
- Scales, penalties and permits are state business. Which vehicles must pull in, what an overweight ticket costs, and who issues an oversize or overweight permit are set state by state, and fines are commonly assessed per pound over.
Whoever loaded the trailer, the manual puts recognizing an overload and poorly balanced weight on the driver. Carry that framing into the test: the wrong answer is usually the one that blames the shipper.
Practicing the weight questions
Weight and balance is recall: the limits, the axle figures, the formula. CDL Prep Exam 2026: Permit runs these inside General Knowledge with a clear explanation after every answer, and flashcards for the numbers themselves.
What to do next
- Memorize the three numbers — 80,000 gross, 20,000 single, 34,000 tandem — and that a legal gross never excuses a heavy axle.
- Learn the bridge formula by its logic, not by heart. Closer axles, less allowed weight; the table gives the figure.
- Know the 36-foot exception and why 12,000 / 34,000 / 34,000 adds up to 80,000.
- Rehearse the two slides: tandems back adds to the drives, fifth wheel forward adds to the steer.
- Read your state's CDL manual, because limits off the Interstate, steer axle rules, scale rules and fines are all state-set.