When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I can't say what the problem might be, the cable could be slipping where it goes into the speedo but I haven't had that problem to be sure. But the '90 fords are definitly cable driven.
cgf350 if you have an automatic overdrive tranny you may have an electronic speedo. If you do the sensor is located in the rear axle housing. I have read several posts that deal with metal shavings and dirt building up on the sensor and causing erratic speedometer readings and transmission shifting problems as a result.