how do i figure out trap rpm with my diff gear?

Hey guys,

I was wondering how can i calculate what gear, speed and rpm i will ge going through the traps at in the 1/4 mile with the gears i have? I have a 3.91 gear right now and will be running a 28 or 29 inch tall tire and my tranny first gear is 3.35 and fourth is 1:1...I want to know what gear i will be in going through the traps with this setup and what rpm as id like to be maxing out fourth around 6k...I also want to try to figure out how this setup would fare with a 4.56 and 4.88 gears- do you guys think that those last two ratios are way too big combined with my first gear ratio and will be me traction problems at the line? (overall first gear will be 15.1 and higher with these but im thinking they will put me at a more desirable trap rpm)...cruising speed is not a problem here, i have an OD..curious to know about off the line with these gear combos and trap speed/rpm...if this seems unclear i will try to explain it more please let me know

thanks
matthew