Frequently Asked Questions about CNG Conversions

Q: Do I get the same fuel mileage on CNG?
A: The mileage will be close, the sequential systems get the best fuel mileage since they are the most tunable.

Q: How many gallons of natural gas will my vehicle hold?
A: Unfortunately the size and weight of a CNG tank is the most misunderstood aspect of CNG conversions.

A 11.4 GGE (Gasoline Gallon Equivalent) type 1 (steel) tank may only have 7.52 usable gallons at 3000 psi. (See explanation below.)

Important: The capacity of any tank is only an estimate. Many variables such as temperature, inlet pressure, condition of the compressor and atmosphere pressure will determine the amount of natural gas in the tank.

A conservative way to calculate how much natural gas you will be able to fill is below. (This is only an estimate and can not always be accomplished.)

  • 11.4 GGE of the tank x 0.66 @3000psi inlet pressure = 7.52 usable fuel.
  • 11.4 GGE of the tank x 0.83 @3600psi inlet pressure = 9.46 usable fuel.

The 11.4 GGE type 1 (steel) tank in the example above is 16” x 49” and weighs 321 lbs.

Tank Types

  • Type 1 Steel: Least expensive, heaviest
  • Type 2 Steel with fiberglass wrap – A bit more expensive. Still heavy.
  • Type 3 Aluminum with composite shell – Lighter, least common, expensive.
  • Type 4 Plastic inner liner with composite shell. Lightest, most expensive.

Placement of  Tanks

  • Trucks – The most common place to put tanks in trucks are in the beds, some may go underneath but will be small due to a lack of room under a truck.
  • Vans – If it is a work van you may but the tank anywhere in the cargo compartment, all lines must be vented to the outside. Family vans will usually put the tanks in the rear.
  • Cars – The tank must go in the trunk, the smaller the car the smaller the space.

As you can see a CNG tank is about twice the size of a propane tank for similar gallons.

Conclusion: To run CNG you must give up trunk, luggage or hauling space.

Q: Will I have a power loss?
A: Most CNG conversions will have a power loss but the fuel savings will make up for it. Aspirated systems may have more power loss than Sequential since they are less tunable. Some conversions have no power loss.

Q: Will I get a check engine light?
A: This depends on the system type and vehicle, as stated above a Sequential system is more tunable and usually has fewer check engine lights. When converting a gasoline vehicle to CNG you are using a fuel that was not intended for that particular vehicle. CNG needs higher compression and advanced timing. CNG still works fine in a gasoline engine but the engine monitoring components are looking for gasoline parameters, thus you may get check engine lights.

Q: Isn’t it illegal to convert a vehicle to CNG unless it is EPA certified?
A: This question as to whether or not a CNG conversion is “illegal” depends a lot on what a persons definition of “illegal” is.

Another question is if something were “illegal” is there any enforcement of that law. Is it illegal for a OEM (original equipment manufacturer) to sell a new vehicle that is not emission compliant? Yes. They test their vehicles so they will run clean throughout their useful life (100K, 120K).

Do we as vehicle owners need to comply with these new car standards? In those states with emission testing a vehicle will be tested annually or semi-annually.

But is it illegal to run a gasoline vehicle on natural gas? Who knows? The shops who do EPA Certified conversions on new trucks seem concerned enough to badmouth aftermarket conversions whenever they can.

Does the EPA feel concerned about aftermarket conversions?

In a correspondence with Marty R. of the EPA I stated:

“Again when someone uses the word “illegal” and kicks people out of their Forum if you mention something they don’t like, I feel they have an agenda. I know numerous people who have left Johns Chat site because of his bias. I know my articles have ruffled a few feathers but I feel that what I write about is useful and is expressing others concerns, Jim”


“It’s not my position to speak for John, but we have had numerous discussions about the intent and the challenges of the EPA certification process. It appears he is using the word “illegal” to be synonymous with non-certified, and “legal” to be synonymous with certified. As you know, the whole purpose of the existing regulation was to define a clear pathway to avoid a possible tampering violation, and as I’ve previously stated, when it comes to EPA making a determination if there is a tampering violation, that’s our enforcement office’s call.”

Marty again leaves it up to the “enforcement office” to make the determination as to whether a CNG conversion is “tampering”. I have written the “enforcement office” and am still waiting for their answer.

Is a CNG conversion illegal in and of itself? Does a vehicle run dirty if it is converted to natural gas? Does the EPA have CNG conversion cops running around handing out fines?

The answer to the first question is no, the answer to the second question is maybe, but the answer to the last question is a big NO.

On numerous occasions I have specifically asked if the EPA “enforcement” division has ever prosecuted or fined anyone for a “illegal” CNG conversion and the answer is always “no”.

Additional Information:

4 Responses to CNG FAQ

  1. John Wood says:

    can I buy new engines to put in my toyota tacoma trucks that are already cng certified. Why not just buy a new motor thats already converted or is there nothing available? I have 21 tacomas

  2. keith says:

    What about vehicles that are old enough to be exempt from EPA regulations such as street rods or pre-1967 Clean Air Act cars?

    • Jim Younkin says:

      Most vehicles are exempt from EPA regulations since they they make them impossible to comply with or understand.

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