I Would like to know how I can figure out how much rpm difference I will get if I swap gears.
My current set up is a 350/350, 3.90 gear with 30 inch tires. I would like to go to a 4.56. I go though the Quarter at about 5000 rpm in 3rd. I want to be at about 6800.
thanks.

With the 4.56 you be about 5850. You'd need a 5.30 gear to get to 6800. I figured this using 114.468 mph, this is what 5000, 3.90, and 30" solves to. No converter slip included.

You can DLoad Speed.exe (http://www.chevelles.com/shop/programs.shtml) from right here at TC. It takes any three of four factores (mph, tires size, engine rpm, gear ratio) and solves for the fourth. I use it pretty often. You can determine shift drops, converter slippage, and several other interesting things by playing around with it.

Thanks a lot If i put in a 4.56 and drop tire size do you think i could do it?

Hard to say... At the same MPH in 3rd with 4.56s @ 6800, you would need approx 25.75" tall tire I think.

just as an example in my camaro i was running 115 mph trap speeds and hitting my 7000 rpm chip as i was crossing the finsh line with 4.56 gears and 28x10.5 mt slicks using a 10" tci convertor th350 tranny. there was about 800 rpms worth of slippage with this convertor. this is not caculated real world numbers..........

another example alittle far off the gear selection but it will give another idea
my camaro finishes the 1/4 mile dead on 6000 rpms at the finish line with 2.73 gears plus the second gear gearing of 1.52 this would be equal to a 4.14 rear end gear. using a 26.2" tall tire with a 12" convertor. this is at 104.41 mph the convertor is right at about 450 rpms worth of slippage.

even thou you are looking for 1800 rpms more for trap rpm just dont rely on the gearing for getting you there.......because then you will over gear yourself and be out of rpms.....as you get closer to your "pulling power" range say 6800 rpms your motor will rev quicker and you will run out of track before you know it....

need to know what your trap speed is. and need to figure your current slipage from your convertor.

most will say your mph will not increase with gear change but if your under geared now increasing your gear will result in alittle better mph. specially if you are waiting for your rpms to climb to finish the 1/4 mile.....kinda like whats happening to you if yours is really pulling good to 6800 rpms......

Originally posted by iwantachevelle:
Thanks a lot If i put in a 4.56 and drop tire size do you think i could do it? I go through at about 6500. This is with 4.56's, 28" tires, and about 7% converter slip. With no slip it would be 6064. Slipage and tire growth (bias ply only, radials don't grow enough to matter) need to be accounted for.