I have to challenge Pug100's use of the word 'should' in that first post.
The HMRC figures indicate the maximum amounts that they'll accept as legitimate 'expenses' (which don't get taxed). For example, if someone was paid 50p per mile (for under 10,000 miles), the tax man would regard 40p per mile as untaxed expenses but charge tax on the other 10p per mile.
The maximum tax-free rates (i.e. 40p per mile up to 10,000 miles and 25p per mile thereafter) are what MPs receive but most people are paid less than the maximum tax-free figures.
It probably costs about 14p per mile to pay for the fuel in your car. So 28p per mile expenses pays you an additional 14p per mile to cover the extra wear and tear on your car (and the higher insurance premium) because you're using it for business. That's quite generous compared to what some companies offer.
I make part of my living from supervising traffic surveys and pedestrian/cyclist questionnaires on behalf of local councils and national transport bodies. This involves travelling across large areas of the country. The firm I contract for pay 17p per mile. Some of us are currently campaigning to get this raised to somewhere in the range of 20p to 25p per mile. 17p per mile might not sound particularly generous but most of the rival firms, in the traffic survey business, pay nil.
Chris