Tires: G78 tires, series tires, p205

Question
what is the current conversion from the old G78 tires to the new ex. 205 75 15

Answer
earl,

There isn't one.

The G78's are 78 aspect ratio and they just don't make those anymore.

However, there are close approximations, if you don't need to be exact:

http://www.barrystiretech.com/vintagetireexchange.html

Short version:  

P215/75R14 or P205/75R15


And you will have problems finding those sizes.  If you go into 70 series tires, it starts to get a little funky in that the wheel widths are much wider and I feel uncomfortable recommending 70 series without changing the wheels.  

But if you need an exact fit, then look up Coker Tire.  They sell tires in that size.