What Is Tag Axle in Trucking?
Also called: Dead Axle, Non-Driven Axle, Rear Tag Axle, Tag Axle Truck
A tag axle is a non-driven axle behind the drive axle or axles of a truck, added to carry more weight. It has no engine power going to its wheels. On the Interstate, each State decides whether a tag axle counts under the federal bridge formula. Some States require it to carry a minimum weight.
Last reviewed September 23, 2026 · Facts checked November 3, 2026
Key Points
- A tag axle is a non-driven, or "dead," axle placed behind the drive axle (Hendrickson white paper on the OPTIMAAX liftable 6x2)
- Peterbilt calls adjustable auxiliary axles, commonly known as pusher or tag axles, ones that add to the load a truck can carry when they are down (2024 Model 579 Operator's Manual)
- A 6x2 tractor has a non-driven front steer axle, one driven rear axle and one non-driven rear axle. In a 6x4 both rear axles are driven (Hendrickson)
- The non-driven axle keeps the same load rating as a driven axle, so the tandem's rating does not change between a 6x4 and a 6x2 (Hendrickson)
- Most North American 6x2 fleets use a non-liftable tag axle that always stays on the road. Hendrickson says that can make the drive wheels spin in snow and ice
- FHWA says each State may decide whether a tag axle counts under the federal bridge formula. Some States require it to carry a minimum amount of weight
- On a liftable axle, air pressure sets how much weight it carries. Peterbilt says lowering the pressure moves load onto the drive axles and warns not to overload them
- With a load and the auxiliary axle raised, never go faster than 5 mph. Lower the axle as soon as you receive a load (Peterbilt)
- Adding or removing a tag axle changes the truck's chassis. Peterbilt says its electronic stability control must then be disabled by a qualified mechanic
What is a tag axle on a truck?
A tag axle is an extra axle that is not driven. It sits behind the drive axle or axles. Hendrickson calls it a non-driven, or "dead," axle. It has no internal gearing to power its wheels, but it still carries load. Peterbilt says adjustable auxiliary axles, commonly known as pusher or tag axles, can add to the load a truck can carry when they are down. Tag axles are used on tractors and on straight trucks.
Where is the tag axle on a semi?
On a 6x2 tractor with a tag axle it is the third axle. A 6x2 has a front steer axle that is not driven, one driven rear axle and one non-driven rear axle (Hendrickson). The tag axle is the non-driven rear axle behind the drive axle. Hendrickson says a non-driven axle can be in the forward or the rear position, and that the rear "tag" position is the more common one.
What is the difference between a tag axle and a pusher axle?
Position. A tag axle sits behind the drive axle. A pusher axle sits ahead of it. Hendrickson says a non-driven axle can be in the forward or the rear position, and the rear "tag" position is more common. Peterbilt's dash switch list names a forward pusher axle and a tag axle as separate controls. Its manual also groups them as auxiliary axles, commonly known as pusher or tag axles. Neither is driven. The lift axle page has a table that sets them side by side.
Is a tag axle powered?
No. In a 6x4, power goes evenly to both rear tandem axles. In a 6x2, only one rear axle is powered, and the other is non-powered, or "dead" (Hendrickson). The non-driven axle carries the same load rating as a second driven axle. So the rating of the tandem does not change from a 6x4 to a 6x2.
When do you raise or lower a tag axle?
Only some tag axles lift. Hendrickson says most North American 6x2 fleets use a non-liftable tag axle that always stays on the road. That can make the drive wheels spin in snow and ice. On a liftable axle, Hendrickson says lifting it on a bobtail, or with an empty or lightly loaded trailer, can add traction to the drive axle and cut tire wear. Peterbilt says liftable auxiliary axles should only be raised to improve maneuverability off road, or when the vehicle is unloaded. Lower the axle as soon as you receive a load. With a load and the axle raised, never go faster than 5 mph.
How much weight can a tag axle carry?
Its rating is the same as a driven axle's (Hendrickson). The legal limit is separate. On the Interstate it is 34,000 pounds on a tandem, which is two axles more than 40 and not more than 96 inches apart (23 CFR 658.5 and 658.17(d)). Whether a tag axle counts under the federal bridge formula is up to each State (FHWA). On a liftable axle, air pressure sets the load. Peterbilt says to set it by weighing the axles on a scale. Exceeding weight limits may result in citations.
What happens to the stability control if you add a tag axle?
Peterbilt tuned its electronic stability control for the truck's original factory build. If you add or remove a tag axle, the chassis has changed. Peterbilt says the system must then be disabled immediately by a qualified mechanic. A wheelbase change, a tractor to truck conversion and a steering system change are in the same list.
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Frequently Asked Questions
What is a tag axle on a semi truck?+
A non-driven axle behind the drive axle or axles. It adds load capacity but gets no engine power. Hendrickson calls it a dead axle. On a 6x2 tractor it is the non-driven rear axle.
Which axle is the tag axle?+
The non-driven axle that sits behind the drive axle. On a 6x2 tractor, that is the rear axle of the rear tandem. The steer axle is at the front.
Are tag axles powered?+
No. A tag axle is non-driven. It has no internal gearing to power its wheels (Hendrickson).
What is the difference between a tag axle and a pusher axle?+
Position. A tag axle is behind the drive axle. A pusher axle is ahead of it. Peterbilt names a forward pusher axle and a tag axle as separate dash controls, and also calls both auxiliary axles.
Why is it called a tag axle?+
We found no maker or agency that explains the name. The documents we read describe where it sits, which is behind the drive axle.
When should you raise a tag axle?+
Only if it is a liftable axle. Peterbilt says to raise it only to improve maneuverability off road or when the vehicle is unloaded, and to lower it as soon as you receive a load. With a load and the axle raised, never go faster than 5 mph.
How much air pressure should a tag axle have?+
There is no single number. Peterbilt says to weigh the axles on a scale and adjust the suspension air pressure until you get the axle load you want. Then write down the pressure-to-load setting. Do not use more pressure than the load needs.
Does a tag axle count under the bridge formula?+
Each State may decide for itself (FHWA). Some States require the axle to carry a minimum amount of weight, or the control to be outside the cab, for it to count.
Do tag axles have brakes?+
A commercial motor vehicle must have brakes acting on all wheels (49 CFR 393.42(a)). The exceptions in 393.42(b) do not mention tag axles. For a lift axle, the brakes need not work while it is raised, but they must work whenever it is lowered (393.48(c)(6)).
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