With that 380 F, your going to get maybe 55-60. I bought one of these (Ski-Doo 2001 380F Formula Deluxe) for an EX-GIRLFRIEND, and all she did was complain that it wouldn't go any faster than 55 or so. Your have to remember, this is a entry level, 40 horsepower fan-cooled machine. On a well groomed trail in some cold temps (the fans love the colder weather) 60 would be the absolute tops.
Our 1999 Formula S 380F does 55-65 pretty regularly. At least that's about what the speedo on my Formula III reads when my 110 pound cousin is on the 380 maxed out beside me. The speedo on the 380 is kilometers, so I have never had it opened up while riding alone where I could guage the speed accurately. It's a fun little sled though, just burning around the farm yard I'd rather ride that than my heavy FIII. It's nimble, snappy, and will jump and land like a champ