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You may call it off road diesel fuel, red-dyed diesel, or untaxed diesel fuel. Whatever your name, this fuel is cheaper than regular diesel fuel, dyed bright red, and illegal to use in a road-going car or truck. Here’s how easy it can be to get caught and how much trouble you could end up in.

Is it okay to run off road diesel?

Off road diesel fuel is designated for non-highway use. This means you can pour it in your tractor, construction equipment, or even a generator. But if you’re operating your truck or car on public roads, it is absolutely illegal to run it on dyed diesel.

Red off road diesel fuel pump against a red metal wall.
Diesel gas pump | Marius Matuschzik via Unsplash

The only real fuzzy area with running off road diesel on road is using it to fuel non-highway vehicles while crossing a highway. According to, you can always run off road diesel in vehicles not designed for highway use. This includes farm tractors or specialized construction equipment.

In addition, you can drive tractors or other agricultural vehicles on some roads. This depends on local laws, whether you are conducting farm business, and how far the tractor’s strayed from its registered address. But if you can legally operate the tractor on a road, you don’t need on road diesel just to do so.

Why can’t you run off road diesel on the road?

Off road diesel is cheaper because when you buy it, you are not paying either federal or state highway taxes. But this means that when you use it to operate a vehicle on the road you are committing tax evasion. That’s a serious crime.

Diesel Ram truck driving up a mountain road.
2022 Ram 3500 diesel | Stellantis

The federal government tacks 24.4 cents highway tax to every gallon of diesel. On top of that, states charge between 12 cents and 67 cents.

This may seem like a lot of money, but this tax pays for maintaining your roads. Why do you pay this tax at the pump? This way people who drive more, or who have heavier vehicles beating up the roads, pay more.

If you fill your gas tank with off road diesel, you have not paid the fee to drive on the roads. Therefore, it is illegal to drive down the road.

How do police check for red diesel?

One way police or tax officials can check for off road, untaxed diesel is simply by shining a flashlight or blacklight into your gas tank to look for red dye. Another method is “dipping” the tank by lowering a test strip into it. A test strip can detect even diluted red diesel.

Issuing a speeding ticket through driver's window
Police ticket | MediaNews Group/Reading Eagle via Getty

Red diesel is very easy to detect, even when heavily diluted with “clear” fuel. With gas prices so high, its safe to say authorities will be checking tanks more often.

If you do get caught running dyed diesel you can face a hefty fine. Many states level a $1,000/gas tank minimum fine. These same states charge $10/gallon instead if that’s a greater fine. That is before any federal tax evasion penalties.

According to a forum post on, one driver in Georgia was hit with $50,000 in fines for running off road diesel on the road.

Next, find out the cheapest new diesel truck for 2024, or learn more about off road diesel, watch the video below: