When would I use "might as well" instead of "may as well"?

The expressions may as well and might as well are effectively fixed idioms, so whether you are certain of getting gas or not does not affect matters.

Both You may as well and you might as well are used to suggest a course of action that either takes advantage of a positive situation that has come up, or at least tries to make the best of a bad one. Both can imply a sense of reluctance in the suggester (the as well implying that the course of action of following the suggestion is similar to not following it).

You may as well eat that left-over food - it will go off otherwise.

You have a day off work... you may as well enjoy yourself.

None of your students have showed up. You might as well go home.

In the first example there is no reluctance - it is simply about taking advantage of the fact that there's left over food. The second example can give the impression that there may be other alternatives (cleaning the house, say) that the speaker has considered, but that on balance they have gone for this option - but they may simply be encouraging the listener to take advantage of their good fortune. In the third example, the speaker is reluctant to suggest going home, because the listener will have wasted a journey and may not get paid.

Of course the expressions are not restricted to the second person - as in your question, you can say I might as well... when describing your attitude to a course of action you're considering taking.

So... which one to use? To be honest, they are more or less interchangeable in most cases (and none of the examples above would be wrong if may and might were switched), but here are a few pointers:

• Might expresses slightly more reluctance than may, though the difference is small.
• Because of this, may is more likely to be used when the suggestion is a positive one (such as enjoy yourself); might when it is negative (go home) or neutral (get a car wash).
• Might as well is used much more often than may as well: COCA reports 3787 uses of the might version, but only 522 for may. So if in doubt, go for might.