I think that for this calculation to be reasonable you need to know the actual fuel usage while maintaining 100kph on a flat highway with no wind. I suspect that it is higher than 18 km/l, that sounds more like a rated highway fuel efficiency number which probably includes a lot more than just maintaining speed on a flat level road.
Damn you! Now I'll have to go back.
I thought this was happening to me, but now I think they are two separate accounts. I have 'firstname.lastname@example.org'. I think the other person has 'email@example.com', and some of his acquaintances occasionally send mail to 'firstname.lastname@example.org' instead of 'email@example.com'.