Pointless question to ask a seller imho.
Reason being that even if as Lexx mentions above you're trying to gauge how the owner drives the car he could just lie to you anyway whilst hoping to make a sale. Everyone tells white lies occasionally and I'm sure a lot of sellers, if not most, wouldn't think twice about telling a buyer the car does more MPG then it actually does.
You're not selling the S/ C VX are you JP?