I have a problem with my tacho and speedometer. the tacho says i tick over at about 2500 and when the speedo reads 60/70 it is off the scale but the speedo also reads too low cos when i read 30 i am doing about 40 and 70 i think is about 90. are these related or seperate and how do i cure cos noone seems to know!!!! the car is a 90 iroc z soft top