Top speed issue on S350
 

Today I decided to see would my '94 S350 reach top speed, as it did two years ago[odometer was showing 195 km/h, but the real top speed is 185(115mph)]. Right after I got out of the shop(had a general service), drove about 20 min to get up the oil temp to working temp, and got out on the highway. And I accelerated slowly but the highest I got was 170 km/h.
I know that is not good, but I really like to fix that.
Any help is appreciated.