YMMV is an initialism that means "your mileage may vary." It's shown on vehicle information stickers that provide an estimate of the gas mileage a car gets. Because driving behaviors differ, the estimates can't be precise. So, your mileage may vary. Either know that and are feigning ignorance to make a point or live in a country that doesn't provide that information. I don't know which. You may not need to know your car's mileage, but if you drive, I hope you have fundamental knowledge of how the vehicle works, lest you would be able to accelerate, brake or steer, because that's part of knowing how the vehicle works.