Top speed varies depending on the car you use so it is always a little guessing. But when you know your cars a little and your tracks then you won't find it hard to take a good guess.
i memorize the length of the longest straight on each track, then make an educated guess as to what speed i can reach at a certain horsepower, have to factor in draft too.
not a formula.
i would not focus to much on top speed however. i won a couple of races where i hit the limiter for a good while on the straight but catched up in the corners.
