[da-burb]

If you are not all that worried about if it was right in the first place, all you will need to do is divide your new ratio by your old ratio.

4.10/3.73=1.10 A 4.10 ratio is 110% of the 3.73 ratio. 3,000 rpms with a 3.73 will be 3,300 rpms with a 4.10 ratio. Your speedo would be off by 10%. 60mph would now be at 66mph on the speedo.

3.08/4.10=.75 if your 4.10 gear was 3300 rpms at 60mph a 3.08 would be 75% of the old 4.10 gear or 2475 rpms. your speedo would be off by 25% You would now be doing 60mph when the speedo says it is doing 45mph.

hopefully I didn't turn any of the numbers around, but it should be correct.

Here are a couple of other sites I have used to help figure this out before.
http://www.novaresource.org/speedo.htm
http://www.bgsoflex.com/speedo1.html