If your car can do 35km/h above the top speed limit in the country I'd suggest it's working perfectly.
However, from a technical viewpoint for those considering racing the 550 (on an official racecourse, not in a Coles car park), there are some limiting factors.
1) The engine is governed. It will NOT rev past a certain level, apparently. I've never tried it, but then 130km/h (with caravan on the back, with my puny 4cyl 2.5L diesel) is more than enough.
2) The engine is governed for TWO reasons. First, to prevent over-revving which can be catastrophic anyway. Second, because diesel fuel takes a certain amount of time to burn and there reaches a point where the engine is revving so fast that the fuel hasn't finished burning by the time the exhaust valve opens and the piston is climbing again. It makes no sense to try and over-rev a diesel for that very reason.
3) Gearing. Diesel engines are capable of propelling race cars at speeds over 300km/h (see Wiki article on the Audi R10
here). They still don't rev high - so the difference MUST be in the gearing. The Navaras (even the V6 diesel) is NOT geared for top speed. They're designed so that at marginally under 100km/h (actual speed, not indicated) the engine is sitting at the top of the torque curve climb. Beyond this point in the torque curve, more torque is generated but at a much greater cost in fuel. The most efficient point to operate the engine is at the top of the steep torque climb - in the 4cyl that's 2,000rpm and in the V6 it's 1,700rpm. If you want your car to go faster you need to change this gearing - there's no other alternative.
I do believe that Eastern Creek Raceway allows street vehicles on the track (for a fee). You could enquire with them and legitimately see what kind of top end your car's capable of.
