Had a tune up at 115,000 miles. I just replaced an O2 sensor on the passenger side two weeks ago. I am not sure if the odometer is correct with the tires. Is there a way to make sure the speedometer/odometer are correct without having to pay a lot of money to get it checked? Also, my oil pressure is reading low, but I dont have any leaks and I recently replaced the Oil sending unit with AC delco and its still reading low. Do you think I could have an electric problem that's causing that issue and also causing me to use too much fuel?