Join Date: Aug 2002
Location: Brandon, Florida
Your VIN doesn't normally list your axle ratio. Per my '95 shop manual, the modern VINs break down as:
1,2,3 - Ford
4 - Restraint Type
5 - Ford Passenger Vehicle (versus Lincoln, Mercury, other, or non-US cars)
6,7 - Body Style
8 - Engine
9 - Check Digit
10 - Model Year
11 - Assembly Plant
12-17 Production Sequence Number
If your car still has the original Certification Label on the door that might tell you the axle code that you could look up.
The jumping might mean that the cable is kinked somewhere, or it might need to be lubed. Where it goes into the back of the gauge might need to be lubed too.
Your tire size may also be throwing off your speedo. If the tires are a lot bigger than what came on the car originally that will make the speedo read slower.
Hope that helps. I don't know a lot about the 80s models but maybe that can get you started.