[AmigaBlizzard]

This is just based on assumptions.
Can you be absolutely sure that Rockstar implemented the exact same speeds as their real-life counterparts?
Even with a value of 180.0 (which can be proven nicely), the mower has a speed of over 40 kph while their real-life counterparts have a speed of about 13 kph (at least mine does).
So with your value, it would reach speed of nearly 80 kph, which doesn't seem real to me.

I too tried to find tha magic number and found 180.0 to be the most realistic.

I just created 2 checkpoints at the highway around Las Venturas.
They were both on the vertical part of that highway at the right-most part of Las Venturas and they were exactly 1000 units apart (1000 units = 1000 meters).
I drove the Elegy at top speed and drove through those checkpoints in a straight line and I measured the time between entering both checkpoints.

So I knew the distance and time required to cross that distance and I could therefore calculate the speed of the Elegy.
From that speed and the velocity value used by samp, I calculated the magic number and found 180.0 to be very accurate.
When I used the magic number of 180.0, my speedometer displayed the calculated speed I found earlier using the distance and time between both checkpoints.

Your value was found based on assumptions (where you assumed Rockstar used the real Ferrari F40's speed and implemented it without changes), my value was based on real measurements ingame.
If you used the mower's real speed (13kph), you would have found the magic number to be around 60.0 or something like that.
So using assumptions isn't going to give any accurate value.