For instance, their information on my GTA is incorrect. They claim that is currently in group F, which is correct.
They then claim that next year it will be in group K, and group M the year after. This is not true. There is no gradual introduction to the new rates, my car will be in group M from next year, more than doubling my road tax.
In fact, the truth of the matter is all held here: http://www.hm-treasury.gov.uk/media/2/5/bud08_chaptera.pdf
In table A8a.
The first three columns can be used to check what you will pay this year.
Next year uses columns 1, 4 and 5. 2010 uses columns 1, 4 and 7.
Column 6 only applies if you buy a brand new car during 2010-11.
So the GTA, at 293g/km is currently in group F (as it is pre-2006 - see footnote 2) and costs £210.
Next year it will be in group M, costing £440.
2010: group M costing £455.