"diff don't affect speedo"!!!!!!!!!!!!
Of course the diff, or rather as asked in this case affects the speedo accuracy, as I assume the speedo is driven from the gearbox output.
Changing ratios will then affect your speedo accuracy, if you put a higher ratio(lower number) diff in there the speedo will under read,
look on www.kabamus.com/garage/gears.html
On this site just put in any combo of gearing you want, tyres sizes, gear ratio, diff ratio etc and get the exact answer.
tabetha