83 300SD wont go above 65
 

I got on the interstate and car ran fine at 78mph then it would not go above 65.

Thanks

Fuel restriction...replace primary and main filters. If that doesn't do it clean tank strainer.

78 mpg... that's just astounding!

I've never seen that kind of mileage from a 617-powered car, even on ebay.

But yes, you'll probably find that the clear pre-filter is visibly clogged. I always carry spare fuel filters with me. The car usually goes from driving fine to sluggish very quickly when a filter clogs.

I changed both filters today. I have not had a chance to drive on Hwy again but car is more peppy. Primary filter looked bad, paper filter inside looked collapsed.

If you still have the problem swap the position of the Fuel Inlet Hose with the Fuel Return Hose. That bypasses the Fuel Tank Screen. If that works your Fuel Tank Screen is restricted.
In order to do the above you need 1/2 Tank of Fuel because you are drawing Fuel from a higher positon in the Fuel Tank.

if your filter has a PAPER element then you have found the problem... GASOLINE primary filters are paper... diesel should be plastic or steel mesh...