85's have the VSS on the back of the speedo, this may be the electronic gizmo you've heard about, I believe that the VSS is elsewhere on earlier models. Other than that the speedometer is the same as earlier models (except the 85mph versions of course).

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k