Supercuda's on the way back...after18 years!

Question for you guys..

I'm trying to approximately calculate what rpm my engine will run if I switch to 3.55 gears. I am running a 4 speed with factory gearing and a 255/40R 18 rear tire. Seems that I saw a formula on here for calculating this. This will help me decide if I am going to run a GV overdrive with my present 3:91's or swap to the 3:55's .
Thanks for any help!!