The 89 - 93 240sx is not the same as the s13 mad Indian. The american 240sx is closest to the Japanese 180sx.
The most noticible difference between the 180sx and the s13 may be the lights, but both have different front ends and the 180sx can also have either the ca18det or the sr20det. (People in america prefer to do the sr20 swap because it's easier to get them and get parts, but the ca18 is regarded by many as a stronger block)
But basically it all comes down the the front end and body type. The 180 in japan is a fastback with flip up lights. The s13 in japan is a coupe with non flip up lights.
Like I said before, the only real s13s in america are where people have done front end silvia conversions with the sr20det swap on a coupe version of the 240sx. Do the same on the fastbacks and the car is then what is commonly referred to in japan as the sila-80. (half silvia, have 180)
People in japan do it on 180s because they like ideal weight balance of the 180 and the non flip-up lights. Since both the s13 and the 180 are built on the same platform, it's also a very common thing to do if you crash your 180 and want to make it look like you really wanted to do the sila-80 conversion. =P
But all said and done, you could say the american 240's can pretty much be called either the s13 or the 180sx.
But yeah, if you wanna be cool and call it by the Japanese name and you own a fastback, you'd say 180sx and not s13.
Soujir0
There I go making a big deal out of nothing again. =P