Tires: Tire Size, rim diameter, fitting questions

Question
I have a truck with 205/75-15 tires on it now. How big of a tire can i put on it with out having problems? i have look at 215/75-15 and 235/75-15 and the 235/70-15. Or can you explain what the numbers mean? I have been told that a wider tire is ok and will not throw off my speedometer

Answer
235 = Section width in millimeters
70 = Aspect ratio
R = Radial construction
16 = Rim diameter in inches

Width does matter.  If to wide, in will scrape the car / truck fender / wheel well, or not fit the rim.

To answer if it will work, contact a tire dealer.
If you tell me what you have, Make, Model, year, 2 or 4 door, 4x4 or 2wd? I can tell you.  I can only answer fitting questions up to 1998.
John