adjustment for tire size

[Cinpro]

there is a tire size calculator for the J-body cars over at J-body.org that will help you to choose a tire size closest stock for when you choose a plus size rim. how ever I don't think it takes into account if you get a wider rim. for example the stock width is 6", and a 195/75-14 tire, I now have a 7" wide rim with a 225/55-16 tire. my "gas mileage" is off since the change.

How can I get a more accurate number for the computer to use? I know there are settings in the editor for the speedometer to change things. I was thinking that maybe I could measure the outside diameter of each tire and compute new numbers from there. but what would be the formula for doing that?

[planethax]

They should be close,
790 Revs/mile stock compared to 783 Revs/mile new size.

New size will be .9% slower so speedo would would read 100kph when actually going 100.9 kph