UI calcs

Current opt-in. In the engine statistics tab, the octane calculation is whatever is set minus octane margin (eg, 91 RON - 7.6 = 83.4) but for the car selection menu, it will show 91 + 7.6 = 98.6

On the exhaust tab, if you set the diameter in millimetres, the tooltip shows in inches, but tagged as metres (eg 31.8mm, tooltip says 1.3 m)