IZELJIC, the simplest way to check the accuracy of your speedometer (which I take to be at least part of your basic question) involves a stopwatch (cheap at Radio Shack) and a freeway with mile markers at a low-traffic time. Ideally, pick a stretch that's fairly straight and flat. Time yourself over two or three miles at a steady speed and do the math. Arithmetically convenient speeds are 60, 72 and 75 but any speed you can maintain consistently will do. For the most precise results, do several runs at different spots and take the average.
As a point of reference, my 560SL turns 2110RPM at 60 mph, according to the Road & Track test of the model in 1989. My speedo incidentally is spot-on at 80, both by stopwatch and GPS; most of my other cars have indicated anything from 77 to 81 at a true 75. Useful to know when maximizing time efficiency while minimizing ticket risk.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)