I've enjoyed the GT endurance races in Forza 6, e.g. the Road Atlanta 100, but noticed that keeping an eye on the fuel gauge while driving was a bit distracting. Have also seen people complain about running out of fuel, for instance the Nürburgring 125 can be challenging since the laps are so long. I was also curious about how worn the tires get in these races, it's noticeable as the race progresses, but how worn are they when it's time to pit?
So I decided to do a bit of simple research, run the Road Atlanta 100 race with each of the nine available cars. I've recorded the results in this Google spreadsheet (which allows comments if you spot an error). BTW, you saw it here first!
Short summary: the Audi R8 and Ferrari 458 run out of fuel before the halfway point, so you'll either have to conserve fuel or pit twice. All other cars complete 22 or 23 laps. Tire wear is typically around 30-40% at that point. I noticed that tire wear starts playing a part around 10%, so on the Atlanta circuit it'll probably be around lap 8 or so.
I haven't yet extrapolated from these results to the other circuits, but it could be possible. When I ran the Nürburgring 125 in the Viper I could pit on lap 4, making for only one pitstop. Two other cars I tested on Road Atlanta lasted about the same time or longer than the Viper: the Corvette and the McLaren. Depending on your driving style they could therefore be good choices on the 'Ring. Not sure about Catalunya and Spa, if I remember correctly I did both of those in the Viper and needed one pit stop.