a site worth a look at would be
http://www.1010tires.com/tech.aspexplains alot and it will even do a search and show you what tyres are available in the size your after.
warning HUGE rant coming
okay sticking with what your asking
1" = 25.4mm
7.5" = 190.5
BUT!
look at it this way the standard wheel width on my car was 6" and it ran 195s with very little tyre bludge.
the way i seen it (untill recently) was well if i increase the rim width by 25.4mm i should be able to increase the tyre width by the same amount.
so with that in mind i bought 225mm wide tyres for my new 7" rims, thats an increase of 30mm, i wanted a bit more bludge to stop gutter rash(worked a treat) and wanted the wheels to look PHAT (also worked a trear). however with the extra buldge the car feels less stable on corner turn in as there is alot of tyre roll and since i had them on 15" rim i had to go 225/50s which meant a larger running diameter, making the speedo read 5% out, the diff ratio drop, and the tyres to scrap on major pumps or road surface undulations.
so to cut to the chase because i could write a two page essay on this.
what size rim is it. diameter and width
what are you looking for.... -style? (if so what type, drifto, drag, racey)
-performance?
what are you willing to spend on tyres?
the site i mentioned should help you with the difinitions and mechanics of it all, if theres something your still not sure of just ask, ill tell you what i know and theres others on here that know a hell of alot more..
if i were you, and those were 15" rims i would be gettin 215/50R15s but that leaves you with one tyre choice thats a road legal club race tyre.
if i were not looking for a performance tyre i would go 205/50R15s that leaves you with alot of choices AND there wouldnt be that much of a difference in performance (what the 215s gained in width it may loose in side wall buldge MAY)
if i was looking for some thing that looks PHAT i would go 225/45R15 (not 225/50R15 as i did) though that leaves you with about one tyre choice also which is also a road legal track tyre.
if they are 16" diameter rims. well thats a whole other ball game...