hi, first post and have a question re my truck

Hi all, I have only recently bought my first truck and am driving as a sub contractor. I was googling for a possible solution to something which is plaguing me and my truck when after having no luck decided to try a forum.

The truck I bought is a Mitsi FM618 traytop 1996 model. The truck is in great condition, has only done 370000 kms. It also has a 1.5 tonne tailgate on the back. I noticed when doing any long trips on open roads that the truck really only ever gets to about 85 kph. If I get a tail from another truck or am going downhill it can get to 90 but thats about it.

At first I just assumed it was the tailgate acting as a big wind drag but in discussing this with other more experienced drivers they assure me that the tailgate although might have some impact should not be causing such a limit on speed.

Incidentally when I had a job which required me to travel north of port augusta I was struggling to get it above 70 through the hills and into a head wind and the engine was getting hot. Frustrated I extended the tailgate out (yes it was probably illegal but I was concerned about the engine and time - i just wanted to get back close enough to port augusta and in mobile phone range) and it made a difference. The engine ran cooler and I was able to get thr truck up to 85-90 again until I got close to pt augusta and put the tailgate back up.

So obviously it has 'some' impact but even with it extended out it wouldnt get passed 90.

It has a 6 speed manual gearbox..the diff seems original mitsubishi. The truck runs fine - it just feels like it needs another gear. When I got it serviced by Daimler I mentioned it but he had no idea what may be causing it other than to say its pretty old. I am not sure that even with its age that it should not be able to get to at least 100 whether loaded or unloaded.

I was wondering if anyone might have some thoughts on it? When it was serviced the mechanic service sheet did mention a leak in the exhaust system somewhere..could this be the cause?

Cheers