Right Bud,
This takes some explaining! 7.50 is a metrified notation of an imperial size, so what it actually means is seven and a half inches! If you then multiply 7.5 by 25.4 (there being 25.5mm to the inch) you end up with 190.5.
Now it is assumed that the aspect ratio of these tyres is 100% so to be absolutely accurate the metric tyre size is 190.5/100r16. Now you try buying one! For a start the nearest width generally for sale is 195, but you wont get a profile section of 100%. So to keep the same circumference you need to increase the width and decrease the profile. Generally speaking 235/85r16 is good to within a percent or two (so your speedo will not be too far out). However if you are increasing the width by 45mm you need to make sure your rims are wide enough.
Rims should be about 70% of the tyre width so if you go for the 235s, the rim should be about 164mm or 6 and a half inches, you could go up to 8 inches. Chances are that you might have 5 and a halfs with your 7.50s!
Hope this helps but if you would like to check out the facts, with some handy calculator tools look here:
http://www.carbibles.com/tyre_bible.htmlLater...
Paul