at 120km/h how many RPMs is the engine doing

 

At 75mph his RPM should be right about 3,000.
Replace the fuel filter, behind the drivers rear wheel bolted to the frame.
Check your air filter, replace if dirty.
Other than that, like Urban Panzer said (he's in Luptin) she is goverend to 120KPH.
Here is the States they are goverend to 110MPH.
Tiy could also look into replacing the VSS (vehicle speed sensor)

 

i know exactly, thats why im bothered

 

On the factory specs plus many road tests back in 95 with the 3.9 with that brick of a body many came up with 104 or 105 mph top end due to the lack of power and aerodynamics.

I now have a 4.6 with a cam, cold ram air, restricted heat to the intake plenum for 73 degrees air temp,(a 3/64" or 0.047" drilled plug in the hot water hose), RPI Tornado chip, 4.6 Mass air meter, 4mm larger ram pipes. A 2" aluminum spacer I milled out sits below the plenum base plate and intake for a longer intake track lowering the max torque rpm zone. Intake track to the heads ported out, bigger exhaust valves with ported to the headers, 2 1/2" CarSound high flow cats, Borla exhaust. Front lowered 3/4" rear 1 3/4", heavy duty springs, WhiteLightning anti-roll bars from Oz. 260% rear and 190% stiffer front bars. Stiff shocks all around. This is a street vehicle only with poly suspension joints no rubber. The chip is built for me to a 6,200 rev limit.
At 35to 36 rpm's per/mph in 5th I have seen 4,450 with a clip on Sun tach that comes up at 125 mph, on radar best in a 50 degree temp day was 127 mph by a police radar gun at the airport runway late one night. Yes this was a active duty radar gun and a hot rod personal cop friend that gunned me both directions. On mountain roads in the twisties is where I have the most fun. Up next a 5.3 litre block with more cam.
This thing feels solid as a rock at speed, no where like it was when stock, stock I had it up on two wheels a couple times. A 'ex Porsche autocross driver here so a shock the first time hanging a corner in the D1, two wheels but not on my lid.
I have no speed limit governor. Fun to spank the new Rovers but I do not tackle the supercharged RangeRover. A larger custom full brass radiator was added plus a pre-oiler. On my 68 F250 it is up to the duty for those Supercharged Range Rovers, at 520 cubic inches, 602 torque, 483 hp at the crank on the dyno with a Mass-Flo MPI as I Tig welded bungs to the intake for injectors. On the Rover I hated the 3.9 Disco, only reason I purchased it was because of the 5-speed, automatic no way. Years ago we put a Rover 3.5 litre in a 62 P1800 Volvo coupe, I always belived in that alunimum motor. By the way the 68 F250 tops out at 152 mph, 2 mph over my projected motor target build. Top speed is in my family for 4 generations.
Nuff ramblings about me, I must state a "PROPERLY BUILT UP DISCO IS SAFE AT SPEED" for a street vehicle. Stock suspension for full off road function is a death wish with speeds above 100 mph. Dodge and chebbies try to blow past me in the mountain twisties with their big tin ego boxes that's when the fun begins, sand bag for a few miles then pass them on the outside lane into a high speed turn. Their look at me, PRICELESS! Sorry had to speak up on the OP plus the replies. I will not be one of those people that stick in a 350 chebbie motor. Here in California I must spend app 35 minutes to convert back to pass smog then back to the power setup. Cheers. CJ..........

 

I never go fater than 70mph in my DI, I leave the speed for the Volvo.