Computer problem
1997 Toyota Rav4 4 cyl Two Wheel Drive Manual 114000 miles

My son's 1997 Rav4 speedometer quit working. The odometer and cruise control still work. The speedometer will work up to about 30 mph then it goes the other way the faster you go. At 60 mph it reads 0 MPH. You can set the cruise at any speed and it stays at that speed. Can I assume that the speed sensor is still working and the speedometer is at fault? I aks because I am planning on buying a new instrament cluster and don't want to waist money if it can be something else at fault.