When a fuel delivery truck pulls up to your generator and transfers fuel, there's a good chance the diesel going into your tank is red — not the clear amber color you'd see at a gas station pump. That red color isn't an accident. It's the government's way of marking a specific class of fuel that's exempt from road taxes, and understanding it can save your operation a significant amount of money every year.
What Is Dyed Diesel?
Dyed diesel — also called red diesel, off-road diesel, or untaxed diesel — is standard ultra-low sulfur diesel (ULSD) that has been treated with a red dye before it leaves the refinery. The dye is added by the IRS's requirement to mark fuel that hasn't been taxed for highway use.
Chemically, it is identical to the clear diesel you pump at a gas station. Same energy content, same combustion properties, same engine performance. The only difference is the dye — and what that dye represents legally.
Red diesel = regular diesel, minus the road tax. It's cheaper because it's not taxed for highway use. It's legal for generators, equipment, and off-road applications. It's illegal in on-road vehicles. That's it.
Why Is Red Diesel Cheaper?
Every gallon of clear diesel sold at a pump includes two built-in taxes that fund road infrastructure:
- Federal Excise Tax (FET) — 24.4 cents per gallon, collected by the IRS
- State Excise Tax (SET) — varies by state, typically another 20–40 cents per gallon
When diesel is going to be used off-road — in generators, farm equipment, construction machinery, forklifts — there's no reason to tax it for roads those machines will never drive on. So the government exempts it. The red dye is how inspectors verify at a glance that the fuel in a given tank is the untaxed variety.
Per gallon, every time. Federal excise tax that doesn't apply to off-road use.
Colorado state excise tax on diesel, also exempt for off-road fuel.
Per gallon vs. pump diesel. On 500 gallons, that's $150–$450 saved per fill.
For a large facility running a 500-kilowatt generator through an extended outage, the difference between red diesel and clear diesel can easily be several hundred dollars per fill. Across a year of scheduled top-offs and emergency fills, the savings are substantial.
Who Can Legally Use Red Diesel?
The IRS is specific about this. Off-road dyed diesel is legal for:
- Backup and standby generators — this is the primary use case for most of our customers
- Farm and agricultural equipment — tractors, combines, irrigation pumps
- Construction equipment — excavators, cranes, compressors, light towers
- Industrial machinery — forklifts, industrial boilers, manufacturing equipment
- Refrigeration units — diesel-powered reefer trailer units (when stationary)
- Heating oil — residential and commercial heating applications
The common thread: none of these vehicles or machines are driving on public roads. The fuel is being burned off-road or in a stationary application, so the highway tax doesn't apply.
Who Cannot Use Red Diesel
This is where people get into serious trouble. Dyed diesel is illegal in any on-road vehicle — trucks, cars, buses, anything with license plates that drives on public roads. The IRS and state tax authorities actively enforce this, and the penalties are steep:
- Fines of up to $1,000 per violation
- Back taxes of $6 per gallon of dyed diesel found in an on-road vehicle
- Enforcement agents can pull fuel samples from any vehicle's tank at any time
The red dye doesn't fade or dissipate — it stays visible in the fuel system. If an inspector finds red fuel in a road vehicle's tank, there's no arguing your way out of it.
If you run both a generator fleet and road vehicles, make absolutely sure your fuel management process keeps these tanks separate. Never use the same nozzle or container for both without a thorough flush — residual red dye in a clear diesel tank is enough to trigger a violation.
Red vs. Clear Diesel — Side by Side
How Do You Get Red Diesel?
You can't pull into a gas station and pump red diesel. It's not sold at retail — by law, it can only be distributed through licensed bulk fuel suppliers who deliver directly to off-road equipment and tanks. This is why on-site fuel delivery exists.
When Diesel2U delivers to your generator, we're delivering dyed diesel #2 by default — the correct, tax-exempt, lower-cost fuel for your equipment. You don't have to specify it, track down a supplier, or haul fuel yourself. We bring it directly to your tank in a DOT-compliant metered tanker, and every gallon is documented.
If your application requires clear diesel for some reason — certain equipment types or specific compliance requirements — we can accommodate that too. But for the vast majority of backup generator applications in Colorado, dyed diesel is the right call.
Does Red Diesel Perform the Same?
Yes — with one practical consideration for generators specifically. Because red diesel is only available through bulk delivery and tends to sit in generator tanks for longer periods than fuel in vehicles, fuel quality management matters more.
Diesel — red or clear — degrades over time. After 6 to 12 months in storage, it can develop water contamination, microbial growth, and sediment that clogs filters and prevents generators from starting when you need them most. This is true regardless of fuel color.
The fix is regular fuel testing and polishing — a process that filters and restores stored diesel to spec. If your generator has been sitting with the same fuel for over a year, there's a good chance that fuel needs attention regardless of whether it's red or clear.
Need red diesel delivered?
Diesel2U delivers dyed diesel #2 directly to your generator tanks across Colorado — on a schedule or when you need it fast.
The Bottom Line
Red diesel is regular diesel without the road tax. It's cheaper, it's legal for generators and off-road equipment, and it's only available through bulk delivery — not at the pump. For most backup generator operators in Colorado, it's the default fuel and the right choice.
The savings add up fast. On a 500-gallon fill, you're looking at $150 to $450 less than you'd pay at a gas station — and that's before you factor in the cost of transporting fuel yourself versus having it delivered to the tank.