i have a boost guage on my steering cowling it displays reedings from -20 to 20 would this be in psi or bar i though the max should be 1.2bar, when it does boost it goes up to 15 and i thought it was 1.5 bar but is it a different reeding

can some one explain

 

Your gauge reads in psi, so when its on boost it is showing 15psi which is not much less than 1.2 bar as 1 bar is 14.2 psi. The -ve section is when the car is sucking air in like a car without a turbo.