That sounds about right for your top end w/ no mods
I put a biTurbo on mine and jetted up to a 68, and it boosted the speed about 7mph. A new air filter would help to prevent 4 stroking, but I just drilled some holes in mine and it works great.
does your gas cap have a measuring cylinder on it?
on mine it says
1 to 25
2 filler caps per 1 quart of gas
I dont know how much the cylinder holds.