The instruments in your Freightliner are not like instruments in an auto.
Each instrument is connected to a common data bus, and selects the data it needs from the bus. You can actually unplug the tach and speedometer, swap the connectors, replug, and they should still work.
Go to http://ww2.justanswer.com/uploads/go...44_sbu5412.pdf
and download Freightliner Service Bulletin 54-12. It has a section on troubleshooting the instruments. After following the steps in SB 54-12, if you still have trouble, call the Freightliner Help Desk at 1 800 385-4357. Have your VIN number available when you call. They've been very helpful to me.
FYI...I've seen my speedo coast down from 65 to zero while the VMSpc display still showed 65 coming from the data bus. At the next stop, it started working again and no more failures.
From your description, I suspect a dirty connector so swapping the cable between the tach and the speedo might be a good first step.