W124 conversion from manual to automatic

[Chlippo]

Hi,

I got a 1991 300E and at my last research i discovered that the car was a 5shifts manual car converted to automatic before i bought it. I am noticing that the car doesnt go up more than 190km per hour. I am wondering if i have to change the differential or other parts in it to be fully converted to automatic since the manual W124 has a 5shifts and the automatic is just 4 gears. I feel that one gear is missing, the car is running 34 rpm when speeding at 100km which is find pretty much high. And the rpm keep going up faster than the speed which makes the car do 45 rpm for just 130km per hour.
what might be the problem...differential or other parts should be changed?
what about the speedo?