I have a 2002 Audi A6, it has 145k miles
Not all the time but sometimes when it gets cold outside (meaning 20 or lower!)
When the car is started it smells strongly of gas inside.
When the car is moved, you can see a large puddle from underneath!
Any ideas as to where to start or what to fix?

 

well if a gas puddle is underneath the car, that probably means gas is leaking from a fuel line. start simple and look at the fuel filter. chances are the fuel smell is coming back through the hvac because the fumes linger in cold air

 

Check the high pressure line under the hood. Mine was leaking at the line leading to to fuel regulator. Only on cold mornings.

 

In very cold weather, rubber will contract and it could be shrinking just enough to result in a leak under fuel pressure. A slight tightening of the fittings may help.