For the second question, people who use kilometers tend to use liters/100 km, rather than distance-per-volume, and use the general term "consumption" for that ratio. Applying that to the first question would be YCMV, which is unit-agnostic, but googling "your consumption may vary" returns results using the "mileage" version, so I'm going to say no-one uses that.
We changed to the metric system here in Canada when I was a kid, so like a lot of people my age I use a strange mix of metric and Imperial units, depending on context. Certain things I'm used to thinking of only in metric, other things only in Imperial - meanwhile there are yet other things I can flip between either unit system.
I can easily switch between using miles or kilometers, but to your original question I would only ever use "YMMV", and have never encountered the use of a metric version of the phrase...
And to your second question... what is this "gas" thing you speak of?