Just guessing wildly here, and I assume you've tried this... but have you tried look in the depths of your system settings? Maybe there's some localization settings for Temp/Pressure/Distance?
When you change Bars to PSI you will also change Centigrade to Fahrenheit and miles to Kms.
Can't have it both.
The iPhone has an app for changing bars to psi - try that if you have one.
At the moment it reads bar, centigrade and km's, doesn't make sense 2/3rds of it is in metric already, psi is the only non metric measurement i want. There must be a way!
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Now you have me confused. Bar is metric. You have everything in metric because you have it set for bar, centigrade and kilometers. If you want PSI, you will have to use U.S. for everything (e.g. miles, fahrenheit, psi.) Like pastorwug said, I don't think you can choose a different system for each readout.