KCF3712 welcome, looks like you have some really good gains there, im still trying to break the 30 mpg barrier myself but im getting close.
I assume the michelin destiny tires are low rolling resistance tires in which case you might get around 6% increase assuming your old tires were just standard tires. This info comes from the wiki. Also i have seen several reports that with low rolling resistance tires the efficiency can go down initially when the tires are breaking in.
Since your new to the site and probably looking for that next good modification you might find the wiki useful if you haven't already come across it
Car MPG Efficiency Modifications Main - EcoModder
It was initially created so new people like yourself dont have to comb through all the threads to get efficiency and how to info.