Volvo: GL940, kmph, fifth gear

Question
I own a 15 year old GL940.It could only go up to 120 kmph.What should I do to improve the speed?

Answer
Wahid,
Test compression and fuel pressure making sure that all is within specs. Make sure the trans is shifting into overdrive or fifth gear. Is the exhaust flowing freely? There could be a clog in the system. Could the engine need a tune up? Clogged air or fuel filter. Restricted fuel line.