gauge seem to read differant depending on road speed or high or low gears

[rs4steve]

When i give it WOT in first and second to the red line the gauge will read 8 bars+ but once im doing say 60mph and floor it in 4th or 5th right to the red line the gauge ill only make it to 4 bars maybe some time 3 , because of the massive differance im not sure wether to set the sc for 1st and second gear or 4th and 5th im running 2 6mm jets and a point 7mm restrictor.
I dont understand how the road speed seems to affect it ?
The outlet is facing the side of the car there was not room to fit itfacing the rear.
i have bleed any air from the pump and tested the preasure at 160psi .
any info would be most helpfull