ok ive been running random math through my head trying to figure what ratio dif i need to top out at about 120-125 and ive come up with 4.4 or so would do that for me. i have a 3.54 in it right now and so i did the math by taking my current top speed presumably 160 and then multiplying it by 3.54 and then dividing by 4.4 and i got a number in the 120's. just wondering if that is an accurate way to calculate the top speed change or not. im thinking it is since there is only one variable being changed and since the wheels, and tranny gears are remaining the same i can consider them a constant or 1 so 1(3.54)(160)/4.4= 120ish works in my world... any thoughts? i have a 280zx tranny the one with the wide gap between 2nd and 3rd if that helps anyone in the know. thanks for your time.~ Jerrod

well looks like i found out the truth about my car... apparently its not a zx 5 speed. it would seem to be a stock 75 5 spd because the gearing isnt matching up with the numbers for a zx tranny. and the nice lil datsun speed calculator where it has the gearing for all those and you enter the tranny you have and the diff and tire size is perfectly matching up for me that i have the stock 5spd and the stock 3.54 dif. so never mind i seem to have found my answer. ~Jerrod

It could still be a 79ZX trans since they are the same as 75-78.

ok ive been running random math through my head trying to figure what ratio dif i need to top out at about 120-125

Man why bother when you can use a tool like this! http://www.geocities.com/z_design_studio/