Heres a good website
http://www.comeseethis.com/gearing/Default.aspx
I was running a 8.5 in my xxx 23/78 as recommended by team losi which gives a FDR of 8.27.
For a 7.5T id go for a fdr of about 9.2 = 22/84 (arpox)
The other advice you have been given is good also, but id say the 6.5t should be more like 20/84
Good luck