Hey Coldwar,
I just got my '79 300D last weekend and thought the same thing. I drove 4 1/2 hrs to the Chicago area to get it and drove it back. The whole way I'm thinking, wow, this motor seems to be reving high for 65 mph. No tach in the '79's either. Well, I was messing around with the gears and noticed that when I put it in Low or 1st, it was in a lower gear than what it starts out in. Based on that I assume I'm reaching fourth when it has only shifted twice from a stop. These things must start out in second, because the Low or 1st is [B]really low. I put new tires in it and it runs/sounds better at higher speeds, but still seems too loud at 65. Maybe that's just the way a '70's benz is supposed to sound - but I have read some people on Ebay saying these things will run down the interstate at 80 mph all day - definately not this one. I wouldn't want to run it up to 80 right now. I'm thinking maybe something with the driveshaft or related. It's going in the shop next week for a checkover.
Try putting yours in low and see if it is lower than what you start out in. Let me know if you figure anything out!
