I am looking for an investment vehicle which is
a good proxy for the long-term price of gasoline.

specifically, these are the criteria:

    1.  I can buy/sell it in reasonably small
        increments (less than $10k)  on short notice
        with an Ameritrade account.
    
    2.  If the pre-tax price of gasoline in the US
        increases by 300% over the next 10 years,  the price
        of the security will most likely increase by at least
        300% in that same time.

Note: If the answer is some kind of energy trust, show
that it is not depleting its reserves at too rapid a rate
to qualify for criterion 2 above.