So I test drove a '99 superduty 7.3 today at a local dealership, it ran good but it seemed like the transmission would not downshift at highway speeds. I was going 45-55 mph during the main part of the test drive down a two lane highway, had a few small hills that should cause it to downshift but it didn't. The O/D light was not on at all, and the engine just lugged and the turbo would not wind up at all at this speed when I pressed the accelerator..however the truck still managed to pick up speed but just would not downshift and rev up the engine. Is this the symptom of a transmission issue or was I just not getting into it enough? From a stop the engine has a lot of power, turbo boosts up good and the tanny up-shifts pretty firm through all the gears. Also looked like the speedometer was a little slow at keeping up with the travel speed..it has stock size tires on it. everything is stock on it.

 

I've no experience with the 7.3L, but my V10 is a little hesitant to downshift at highway speed. It just pours on a little more speed without much fuss when I push down the accelerator. The one thing I notice it does at the drop of a hat is unlock the torque converter with the slightest of throttle position changes.