1. If you come across such numbers in the future, and you think they will draw my attention, please add a comment explaining what you have done about it.
2. By miles per gallon, car A is better, but by miles per hour car B is better. You asked what sounds more natural: 'Car A gets better mileage, but car B is faster.'
3. I think it will be good to check with him once, before we do this. This is okay. You might be able to leave out the word once, depending on the situation.