There's been occasional discussions about tire and wheel combinations on our cars...what works, etc. I found this information from the Second Chance Garage about replacing tires and wheels with different sizes and how to do so and keep the speedometer accurate. It doesn't address the wheel offset but it's good information anyway. I'm quoting it verbatim:

1. Measure the diameter of the stock wheel/tire (on the horizontal, please, since the bottom is distorted from resting on the ground). If you don't have the wheel you can find the diameter by looking at the specifications in the owner's manual or the shop manual.

2. Choose the new wheel size you wish to mount and note it.

3. Read the aspect ratio off the tire you plan to use (it's the number after the "/" in the raised portion. Example: 225/60 means the aspect ratio is 60). The aspect ratio is the ratio of the height of the tire to the width, expressed in percentage.

4. To find the height of the tire from the wheel rim out, first find the width in inches (divide the metric number by 25.4 to get inches) and multiply that number by the aspect ratio.

5. Add this amount to the rim diameter to get the overall diameter of the new setup. If the diameter doesn't match your stock measurements, look for another tire that has the right combination of aspect ratio/tread width for the numbers to work out.