Where did you get the "minimum $12,000" figure?  I can't find a source
for a fine that large.

However, see http://www.irs.gov/irm/part20/ch01s19.html, a portion of
the "Internal Revenue Manual (IRM)".  I quote below from that IRS
document.

Begin quote:

IRS=>20.1.10.15.1.1  (01-01-2006)
IRS=>Improper Sale of Dyed Fuel
IRS=>
IRS=>   1. The penalty is imposed if any dyed fuel is sold or held for
IRS=>sale by any person for any use that such person knows or has
IRS=>reason to know is not a nontaxable use of such fuel.
IRS=>   2. "Nontaxable use" has the meaning given to the term by IRC
IRS=>section 4082(b) , Nontaxable use.
IRS=>
IRS=>20.1.10.15.1.2  (01-01-2006)
IRS=>Improper Use of Dyed Fuel
IRS=>
IRS=>   1. The penalty is imposed if any dyed fuel is held for use or
IRS=>used by any person for a use other than a nontaxable use and such
IRS=>person knew, or had reason to know, that such fuel was so dyed.
IRS=>
IRS=>20.1.10.15.1.3  (01-01-2006)
IRS=>Willful Alteration of Dye Concentration
IRS=>
IRS=>   1. The penalty is imposed if any person willfully alters, or
IRS=>attempts to alter, the strength or composition of any dye or
IRS=>marking done pursuant to IRC section 4082 in any dyed fuel.
IRS=>
IRS=>...
IRS=>
IRS=>20.1.10.15.3  (01-01-2006)
IRS=>Penalty Computation
IRS=>
IRS=>   1. For the first violation, the amount of the penalty on each
IRS=>act is the greater of—
IRS=>         1. $1,000, or
IRS=>         2. $10 for each gallon of the dyed fuel involved.
IRS=>
IRS=>   2. For additional violations, the amount in (1)(a) is
IRS=>determined by multiplying $1,000 times the number of prior
IRS=>penalties imposed by IRC 6715 on such person (or a related person
IRS=>or any predecessor of such person or related person).
IRS=>         1. For example, if 50 gallons of dyed fuel is involved
IRS=>in a person’s first violation, the penalty would be $1,000
IRS=>because that amount is greater than 50 X $10. If 150 gallons of
IRS=>dyed fuel is involved in that person’s second violation, the
IRS=>penalty would be $2,000 because that amount ($1,000 X 2
IRS=>violations) is greater than 150 X $10.

End quote.

As I read the above, the penalty imposed by the IRS on a diesel pickup
found to be burning dyed (untaxed) fuel is $1,000 for a first offense,
the minimum fine, unless the pickup's tank capacity exceeds 100
gallons.  Additional fines may be imposed by state enforcers.  The
penalty is nowhere near the $12,000 figure you cite unless the
violations involve lots of gallons, or the offender repeatedly gets
caught and fined.

Still, one would need to burn a lot of untaxed fuel to save enough to
cover a single $1,000 fine.

Bob