we had 4:10's in the mustang in my Avatar,
and it gets 17mpg, almost every tankful.
it is AWESOME. it pulls great.
for a V6, i would do the 4:10's
but if you got these free, and you can get them in free by doing some swaping/trading,
i would put them in, and just run the 3:73's
free stuff has WAAAAY more SOTP