Will toss this in to see if it helps. I bought a 7.3 2.5 years ago and this group has a lot of great advice to keep your truck running! The 6.9 and 7.3 are very similair and share many parts. So 7.3 advice will help you too.
Always add as much color as you can. I see white smoke at start up, I have a hard time starting up, I have a miss or rough idle, I see black smoke when I step on it, I feel a sudden lack of power at 55mph. The more symptoms you share the more this group can weigh in on things to look at or better diagnose.
I have a 88 F250 with a 7.3 non-turbo and C6 tranny but not sure what gears the axle has. But as said above when geared down these are not 80mph cruisers. (Think powerful tug boat, not speed boat.) But if you think about it, follks towing back in 80's and early 90's towed at 55, so it gave them higher RPM's, more torque, and saved the bottom end of the engine. (I have read they do not like heavy loads at low RPM's.) They guy who sold me the truck just used it for pulling a 5th wheel travel trailer, not a daily driver. So he probably did'nt care if he could drive in the fast lane. Know on steep hills pulling my 8,000 pound boat I am in 2nd doing 45mph.
My truck unloaded goes
1) 3,000rpms = ~70 MPH (About as fast as I care to run up the engine.)
2) 2,700rpms = ~63 MPH (My general cruising speed is 63-65.)
I would see if you can get your RPM's up to 2,700-2,900. If not there is likely some good reason (governed.) or you have an issue. (Lack of fuel or good tune.) If you can get higher RPM's per the response above, your tranny could be slipping, or you have a bad torque converter, or simply you are geared for 55mph @ 2,500-2,800 rpm. (That would be a first I have seen on this site, but things happen for a reason.)
1) As stated above perhaps the govenor was lowered to keep their employees from speeding or blowing up the truck. Perhaps someone else can share what happens when going down a hill, does the govenor still restrict the RPM's. I would imagine if governed when you hit the max, the engine will slightly change sounds. If you say I am going to go steady at 45mph you will hear the engine change tune from accelerating to coasting as the injection pump starts to starve the engine of more diesel. I notice this when going 65 on the HWY, when the engine goes form coasting to pushing and visa versa you hear a change in the sound of the engine and feel a little deceleration.
2) As stated above possible fuel restriction issue but I would expect you would feel a odd reaction, like irregular power, surging, or the truck power falling off as it starves then picking up as it gets more. (Governers generally feel like you hit the wall, that you are giving it all its got.) Switch fuel tanks if vans have dual tanks like a F250/F350, that would allow you to determine if the tank pick up or line to the lift pump is clogged or if the fuel is bad. Change the fuel filter for sure which is just good maintanence for a new used truck. (I would expect a govenor would just feel like you maxed out, not starving for fuel.) Also perhaps test your lift pump, as mentioned earlier, if you lack volume of fuel to the injection pump, you are not able to go as fast.
3) Injection timing, when I bought it used I put on new injectors and when I checked the timing it was at 0 degrees at 2,000 rpm's VS 8.5 degrees BTD at 2,000rpm. Timing can also limit your power. (Usually creates other issues like white or black smoke or in my case higher temps.) So not sure if new injectors changed my timing that severe or if the prior owner did not have it timed correctly.
4) Also maybe do a compression and test to see if you have level compression.(Lets you check your glow plugs in the process.) You will read there is no official PSI, just no more than 20% off between the highest and lowest cylinder. Mine all tested between 405 and 420psi to give you a guage, but I would have not worried if that was 340-350 PSI as they would be close together.
5) Folks have also had odd issues from clogged injector rail return lines. So you may want to blow air through your line to the lift pump, and the return line with the gas cap off and listen for air. (I would not do so at 120psi out of a compressor, but increase the PSI slowly from say 5psi to 15psi to see if clogged.)
On another note is is frustrating that Ford did not put a extra digit on the speedo. As folks suggest some service records and online services may have picked up miliage along the way. But often you can tell by the wear on door hinges, seat covers, and overall appearance to gauge if the truck has say 50K, 150k, or 250k. My seats were not ripped and the doors were not loose on the hinges or lock tabs. So I was pretty confident the seller was honest and the truck only had 170K not 270k. Bottom line is now its your truck and is what it is. Hopefully you got a good deal because the seller also knew it would not go over 55.
There are likely threads on this site on how to change the govenor to see if that is your core issue. There are also threads on all the rest above.
Stay the course, read up on other threads, and you will succeed. Read about the general maintenace issues, common problems, and address them. These are relatively simple diesels, parts in comparison with new rigs are cheap, and with a positive attitude you can do most of maintenace, diagnossis, and work yourself.
