I would guess 18's or 19's based on with I did with my MTX when it was basicly stock and what hector has done in his ATX with ALOT of mods....
Ran a 17.3 at 81.83mph on a warm muggy day, stock. Later in the year with a cold air intake and a free-flowing muffler ran a 16.5 at 87mph, good enough for a little 5-speeed, especially when the 3rd gear synco is going.