you know how the speedo waves about now just take the lowest reading as your speed.
Ok i did this years ago with a v8 and 3 54 diffs,if you take off 10mph you should be ok,if you go over 50mph start taking a little more off,i never got a ticket so should be ok..
Get a pal to drive in front at 30mph etc to know for sure
Hi if u search it on google there is a couple off companys do it. They ask you to fill a form out with tyre size and things then tell u how to find the W factor. Then u send the info and the speedo headcto them and they calibrate it i think it said about £60