I work as an environmental manager & I generally remove about 50 tanks a year. These are the large 10 to 20,000 gallon variety. We pull gas, diesel, motor oil & waste oil.
You can reduce the vapors in two ways. Allow the tank to sit, fully vented in the sun for a few weeks or wash it with strong detergent & water (see below)
The goal of either method is to get to less than 25% of the lower explosive limit of gasoline. Your local fire department may be able to measure this for you using an explosimeter.
You can clean you tank with a 4 : 1 mixture of water & Simple Green or similar product. This will break down the gasoline, reduce the fumes & allow you to flush out the tank. The rinse water is technically considered hazardous waste because it will contain traces of benzene. You should dispose of the waste in accordance with local, state & federal law.
