I can see it being a normal question to ask if it was a particular type of car. If it was my car, for example, a normal run of the mill diesel - it's absolutely a normal question to ask in that way.
If I wanted your car and I'd just seen it and decided to ask you 4 questions to qualify whether I'd be interested in seeing it, I certainly wouldn't be wasting one of them on 'whats the mpg mate'. I'd be far more bothered about whether you'd serviced it!