strobe slipper and hv-maxx ?

[mini-mad]

hi everyone!!
i want to buy a strobe slipper for my truggy gmaxx but i have a question :
which gear should i use,now i'm in 13/70,which pinion must i use to keep my speed ?
yes,i know,the hv-maxx has a 3mm shaft but i have a friend who can do a 5mm axle:027: to fit with the mod1 gear.
thanx for your answers:o