how many rpm's will i drop going from 3.91 to 3.55????????????????

I go 80 with my 3.91's, the little 340 loves it and gas might hit $3 this fall

I would not go with 3.55's, you will lose more then you will gain. 3.21 or yeah, 2.94's would make a better highway gear for today's fuel prices. The 3.55's will still burn up the gas but it won't have as much kick when you nail it.