Yeah, the idea of friends here at Fuelly is built around people instead of vehicles. The problem with following only specific vehicles from friends is that it makes managing those friends more complicated. So anytime a friend adds or removes a vehicle, you'd need to decided whether or not to follow that vehicle.
One thing you might do is tune into the Friends Activity page instead of the simple list on your Dashboard. That page lists more detail for each activity by your friends. That way you'll have a better of idea of the fuel-ups you're interested in. Another option is that if you already use an offsite newsreader like Google Reader, you could add only those vehicles you're interested in there. Every vehicle at Fuelly has an RSS feed with fuel-up details. So you could skip adding them as a friend, and follow the feed for a particular vehicle. Just keep in mind that you'd need to stay on top of any vehicles they add or remove.