Actually changing the offset is good way to make your wheel "fill" the fender well - even a same size wheel will look better if the offset brings it out to the edge of the fender. So it's not critical to preserve the same ratio of offset - in fact it may be better to tune exactly.
If you want to customize, first take diameter of old & desired new and divide the difference between old & new by 2 & convert to mm. If your offset was the
same on your new wheels as original, then this would move the outside rim of the new wheel out towards the edge of the car by this much.
i.e. on the rear - go from 8" to 10" - difference is 2"; divide by 2 = 1" = 25.4mm. So by retaining the same offset as original 8" wheel (60mm) the new 10" would move out towards the edge of the fender by 25.4mm.
OK, so all well & good - so why not stick with 60mm offset? After all, some guys are fitting 25mm spacers, which do the same thing? Because then lip comes into it - by decreasing the offset, you will move the centre hub further in still, relative to the edge of teh rim and this will give you a bigger lip - which tends to be very desirable re appearance. This will also move the wheel further out still, so you can't go crazy with wider wheel and too small offset or you'll end up being outside the fender. You also have to recognize that when going wider wheel, with no offset change, your new wheel
inside rim edge is getting closer to the shock, so you may need to adjust for less offset to accomodate that too.
You can just use a rule & measure up how much gap you have now, where you want the edge of rim to end up, what width wheel you will use, how much lip you want and then calculate all this into the equation.
But much simpler - typically you can go by what others have used as a guide for given width of wheel & check out their pics of finished article.
For example, you can see mine
here - the fronts are 38mm on 8" wide and the rears are 45mm on 10" wheels.
See the little explanatory diagrams in
the Wheel Tech FAQ to get good understanding of how offset works.
edit:
p.s.
emdoller said:
....In order to figure out the difference in stock wheels, I took the stock offsets (55mm front on 7" wheels) and (60mm rear on 8.5" wheels) to figure out the stock distace from the hub to the end of the wheel. If my calculations are correct, the fronts are 33.9mm or 1.3" and the rears are 48mm or 1.9" from the hub to the end of the wheel.....
Your math is correct - now see notes above to figure out how increasing width of wheel will change the final new outside rim position wrt to the hub.
Remember for
same offset, just add half the width increase to your calculated number, then adjust again for any change in new wheel offset vs stock.
e.g. your new front wheel is 8" with offset of 40mm -
then the new outside rim edge distance from the hub = 34 mm (as you calculated for OEM wheel) + 12.7mm (wheel width increase) + 15mm (offset change) = 61.7mm, or a change of 27.7mm closer to the outside of fender well.
