The tax band thing works like this:
Personal Allowance Up to £11,000 0%
Basic rate £11,001 to £43,000 20%
Higher rate £43,001 to £150,000 40%
Your company car will be assessed on list price when new and emmissions, e.g. assume it’s considered to worth £4000 to you this will be knocked off your personal tax free allowance so you’ll now be paying tax on your pay as follows:
Personal Allowance Up to £7,000 0%
Basic rate £7,001 to £39,000 20%
Higher rate £39,001 to £146,000 40%
So if your salary is less than £39000 you’ll pay an extra £800 tax. As for fuel in my case I pay for all fuel and then claim back any business mileage I do.