I was wondering:

Lets say I was running 17x7.5, +55 fronts and 17x9, +63 rears with 225's and 255's.

If I got 25mm wider fenders (about an inch), theroretically could I run 17x8.5, +30 front and 17x10, +38 with 245's and 275's?

I figure, by subtracting the 25 from the offset leaves the inside of the wheel at the same location, it just bumps the outside of the wheel out 1 inch.

Please let me know if my understanding of offsets is correct.

 

the offset situation doesnt work that way. those two sizes you have on the 8.5 and 10 widths are actually going to sit about 1.4 and 1.5 inch outwards. you have to remember that offsets work off from the center of the wheel and you have to calculate from there.

if only the offset calculator is up you can see that it is alot different than subtracting from the offset and adding inches.

 

Will this help you visualize your situation?

http://www.1000rims.com/rt_specs.jsp

 

That link is awesome...thanks.