According to Garmin 12-channel GPS, my car is a solid 5% off pesimistic.

My 350 is a little on the low-side too(maybe 2-3%)

80-85 takes 3 seconds in 4th, 85-90 takes 3 seconds in 4th, 90-95 takes 4 seconds in 4th, 95 - 100(these are readings from the speedometer) takes about 3 seconds in 4th, can you say, "locomotive"?

The car is faster than the speedometer really tells. I measured mine, and it is a good 5% off. Turbodiesel, while you were going 115 mph, if your speedometer was off like mine, you were actually going better than 120 mph!

You see my tach, is it similar to what yours was at those speeds? Maybe your car was faster than you thought!

My sister's '87 300D is actually reading too optimistical. At 60, it is doing 57 - 58 mph.

The CDI's are ridiculous off the line too. But they get more sluggish as speeds rise above 80 mph. Its a diesel thing I guess, but the 603 tends to slow down to 4-5 second 5mph crawls after 100 mph. This is consistent until you let off the pedal or hit near redline. It does just go like a locomotive. When you go past a certain point, the boost pressure falls and the power is noticeably less. You'll feel the car restrain itself. This is telling you it is nearing redline.

90 - 100 mph is the weakest spot on the 603. Before or after, it moves quickly. Almost like a transition. After 100 mph, the engine stops growling and almost purrs.