The 124.036 was allegedly goverened to 155mph, however some cars seem to not have the governor (don't know why). Ungoverened, with the stock 2.82 gears, they will reach 164-165mph, gearing/redline limited. With the gears changed taller (say, the 2.62 gears from an R129) the car is drag limited in the 172-175mph range. If you don't believe me, ask some people in Europe who actually play with these things frequently at those speeds. The Cd is amazingly good on the W124. The assorted 6.0/6.5L W124 tuner cars (AMG, Brabus, RENNtech, etc) were mostly capable of 180-190mph due to the extra power *and* taller gears. I found some specs on those once, I can dig them up again and post here if anyone cares.
