Outer Limits 4x4 Board

hi all, i have just turfed the f10a and 4speed out of my 94 wt maruti and fitted a 1.6 mpfi & 5speed, to gain some on road performance and drop my revs at 100. from the number crunching i have done, it used to do around 52?0 rpm @ 100 with the 4 speed and standard 1lt transfer with 4.1 diffs on 205s. same setup now but with the 5 speed should bring it down to around 4500 rpm @100. thing is, i want to get it back to stock vit revs at 100 for economy and noise. so fitting 3.7 diffs will drop it to 4000 rpm which might have to be close enough. so what i would like to know is, is the speedo drive gear on the transfer output shaft the same for all the diff ratios( with a different cable drive unit) or is the whole thing different and not interchangeable?
so you know besides the vit engine and sierra 5 speed it is a completly standard lwb maruti, alloy tray standard suspension, standard tyres, no BL, standard 4.1 maruti diff gears and 1lt transfer. Its the only way the misses will drive it and its not bad fun having a practical sleeper type thing
thanks if you can help,
Jason

up the tyre size =P

its a sleeper!! i want to keep it stock, but thanks for the thought. i know if i put 31s on it it will be apples but that is not what i am after