Volkswagen: Air bag light, speedometer needle, jetta gls

Question
I just recently purchased a 99 Jetta GLS, the air bag light stays on and the speedometer needle fluctuates at speeds above 40 MPH.  Also, the odometer is recording mileage at a lesser rate than what is actually driven(about 10%).  My question is are these problems related and if so where to I begin to troubleshoot.

Answer
sadly due to the complexity of the car's electrics, the speedo is electrically driven. to check the electric signals you would need specialist equipement such as a vag.com diagnostic computer. theses pieces of equipement cost thousands. so your only real choices are to pay someone to carry out the diagnostic checks or to purchase and try componants (which could prove to be very costly). if this is the course you choose, i would try replacing the gearbox speed sensor first, as the next componant which may be at fault is the kombi processor in the dash pod. should the later be the culprit, you would need a specialist or even main dealer to code the immobiliser unit in the dash pod to your keys transponders. from a profesional point of view, i would pay to have it code read / diagnosed by a specialist to prevent wasting money. the airbag light could be caused by a number of factors, ranging from poor connections to faulty airbags. there may be anywhere between 1 and 8 airbags. YOU CANNOT CARRY OUT RESISTANCE CHECKS on any airbag circuits. therefore again it needs checking on diagnostic equipement. if the fault reates to the drivers airbag, it may be a "slip ring" fault. the slip ring is the device which allows the steering to be turned without breaking the circuit. asd this part is constantly moving it is prone to wear and eventual failure.