gears times and money

so i have a 6 speed and stock rear end with 342's. just wonderin if i went to 410s what kind of improvement i could see in my eighth and quarter times? currently at about 70 on the interstate im running maybe 1700 rpm... if i went to 410's what could i expect at this same speed? also how much would my gas milage suffer?

3.42's @ 71MPH = 1600RPM. Your tach may be off a little (like everyones's)
4.10's @ 71MPH = 1900RPM.

4.10's are definitely a better track gear, and totally liveable on the street with a 6-spd - one of these days I'll be going with 4.10's in a 12-bolt for my car.

Much of the improvement with 4.10's would be lost if you can't hook the car - it will be easier to spin the tires with the 4.10's. And in the stock 10-bolt, 4.10's have a reputation for being weak, since there are more, smaller teeth on the ring and pinion gears.

With my car currently getting 1.8-1.9 60' times and getting into 4th gear about 200' from the end of the track, I'm hoping for .3 seconds improvement with the 4.10's over the 3.42's.