2005 Chevy Impala speed reading

I think by your speedometer reading 20 or thirty when the car is off, means the vehicle speed sensor is using power from the battery to do this, you will need to correct this at some point. But I dont beleive you should be worried, as far as the brakes, that squeel 9 times out of 10 is the little metal sensor scraping against the rotor, that indicates the pad is low enough for that little piece of attached metal to scrap and signal low pads. Its early, I hoped I explained that so you can understand.