How many tyrannosaurs in a gallon of gasoline? 

While I suppose that most of the carbon in America's favorite fossil
fuel is botanical in origin, it's easier to get a gut feel for
unrenewability by considering dinosaurs.

So to be more specific, if you estimate the number of carbon atoms in
one full-grown adult tyrannosaur, what volume of 92 octane gasoline
would that represent? And conversely, how many dinosaurs could you
build out of the carbon in one gallon of gas?

The answer is that the carbon content of one tyrannosaur is equivalent
to that in about 460 gallons of gasoline. (You can build 1/460th of a
tyrannosaur using the carbon in 1 gallon of gasoline.)

You are probably well aware that gasoline is derived from crude oil by
separating it out in a refinery. Crude oil, in turn, is pumped from
underground reservoirs which are generally believed to have taken
millions of years to form. As such, oil is considered a
"non-renewable" resource in that we are essentially pumping out in a
few hundred years what took millions of years to form and concentrate.
And, as you note, the current understanding of the formation process
is that oil is derived primarily from non-animal sources--probably
mostly plankton. The conversion process from a tyrannosaur of 70
million years ago to 92-octane gasoline now would be, shall we say,
not impossible but rather indirect. You can find a brief discussion of
fossil fuel origins in general in "Topic #14 Power from the Earth -
Fossil Fuel," part of the Geology 100 notes for the Department of
Geology at the University of Illinois at Urbana-Champaign:

http://ijolite.geology.uiuc.edu/02SprgClass/geo100/Lectures/100_T14_fuel.html

We will, however, ignore any inefficiencies in the conversion process
and imagine that we can convert all the carbon in a T. Rex into
92-octane gasoline. Searching a few websites for a consensus on the
weight of an adult tyrannosaur, one finds a range of estimates between
5 and 8 tons (no point in worrying about whether these are US, English
or metric tons). A reasonable consensus seems to be about 6 metric
tons (6000 kg). A Google search on

://www.google.com/search?q=tyrannosaurus+weight

gives many sites with weight estimates. Of those 6000 kg, of course,
most is probably water. I had a little trouble turning up an accurate
estimate for how much is likely to be carbon. Looking up the carbon
cycle in John Kimball's Biology text:

http://www.ultranet.com/~jkimball/BiologyPages/C/CarbonCycle.html

one finds an estimate of 18% carbon for living matter in general.
That's probably right for a dinosaur to within better than a factor of
two. So, one Tyrannosaur contains about 1100 kg of carbon.

Now, we need the carbon content of 92-octane gasoline. If we
approximate the gasoline as pure iso-octane (C8H18) (see, for example,
"Fuel Chemistry" at

http://www.altfuels.org/fuelchem.html ), 

then carbon represents 84% of the weight of gasoline. The density of
gasoline is about .75 gm/cc. (See, for example, the "MSDS
Hyperglossary" at

http://www.ilpi.com/msds/ref/density.html ).

One US gallon contains 3785 cc (online unit conversion at 

http://www.onlineconversion.com/volume.htm ).

Thus, one gallon of gasoline contains 3785 x .75 x .84  / 1000 = 2.38
kg carbon.

Dividing this into the mass of carbon in our average tyrannosaur, we
can convert one animal into about 460 US gallons of gasoline, assuming
100% conversion of the carbon.

Some useful Google searches:

://www.google.com/search?q=origin+fossil+fuel
://www.google.com/search?q=tyrannosaurus+weight
://www.google.com/search?q=carbon+cycle
://www.google.com/search?q=specific+gravity+gasoline
://www.google.com/search?q=unit+conversion

An excellent answer. Thorough, yet concise. Cites (and links to)
authoritative sources that provide a wealth of background information
for each step of the analysis.

And all delivered within a few hours. Nice work!

I'm just guessing off the top of my head here, but I am pretty sure
you would be getting quite a few gallons of gasoline per T-Rex if you
are just comparing quantities of Carbon. The question presented is
would be similar to how much graphite powder would it take to make a
diamond. The answer would be somewhere near 1:1, as they are made of
the same molecules... It's really what it takes to turn a T-Rex into
the gasoline that is the real question