Here is a quick formula I thought applied

16/43 stock
15/43 modified
 if you divide the # of rear teeth by the # of front teeth you get a ratio.

43/16 = 2.6875
43/15 =  2.8666

now you can divide your two ratios,

2.8666/2.6875 = 1 .06

or 106% of the stock gearing, making you speedo read 6% higher, 100 mph
stock, 106 modified

here are some common gearing changes with the %

16/41 = 95%
16/43= 100%
16/45 = 104.5%
15/43 = 106%
16/47 = 109%
15/45 = 112%
14/43 = 114%
15/47 = 117%
14/45 = 120%