I thought they did it in 82 when they went with the turbo. Any 300D with a turbo should easily be able to go over 85 so that would make sense. Are you sure your speedo hasn't been changed out?

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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