what does std mean for a1998 mustang convertable

I think standard transmission. Does it have a manual or automatic

Standard transmission refers to a manual transmission.

more common as in the transmission, HOWEVER, personally (not a Ford guy), I would call as to thinking they in this case refer to a package option, many STD mustangs are AUTOMATIC, but aluminum rims and such...again I can be way off, but a call to a dealer should satisfy your question.

STD is a program used when dyno testing a car that uses better testing conditions in order to show a higher number than ur car is IDEALLY running...it tests ur car with an air temp of 60 degrees F and a higher air density....the most accurate reading is SAE testing which is what u would request when getting dyno tested becasue it runs at 77 degrees and a more average air density to where most people live....STD dyno testing gives u an extra 4.8% hp reading to make the dyno place look cool and give u better readings

it means manual transmission

http://www.gettorq.com/store/blog/sae-vs-std-dyno-numbers/

STD.....i hope they don't mean sexually transmitted disease

When a mustang is standard it means it is a non-GT model. It may be a combination of options to what ever that year offered. Usually cloth interior, may not have power windows. Etc. best thing to do is go to kelly blue book and search for that car and available options.