my LS400 Lexus 90 speedo-meter reads 80km per hour while am actually making about 120km per hour,what could b

Maybe you're not reading it correctly.

How r u confirming that you're going 120km? Radar gun? Sat Nav? In any event, the most likely cause for such a disparate reading would be incorrectly sized tires. Whenever u change the ''aspect ratio'' u affect the speedometer reading. For example, if ur car came equipped with 225/75/15 tires (225 = width, 75 = aspect ratio which means the height is 75% of the width, 15 = rim diameter) & u now have 225/60/15 is the rotation will be vastly different because the dimensions r so far off. Going up in rim size can also affect readings, as can tire width (again, because it will affect the aspect ratio).

Check out http://discounttire.com for more info on aspect ratio.

Check ur owner is manual for the stock tire sizing (ie. 225/70/15 for example) & then check the size of the tires currently installed. Then check both against the source l posted for more info.

That is probably the speed regulator, either it was set too low or your seedometer is just messed up. Go to the dealer and get more info.

If it is a digital gauge, then maybe it has switched from KM to MPH. Has the mileage on the odometer also reduced?