Here in the US people who buy hybrid vehicles can usually recover most of the cost premium of the extra hardware through a variety of tax breaks. Unfortunately government agencies gain no benefit from this and have to bear the full cost. Over in Britain the government has been buying hybrids for official use.
However, a group called the Taxpayer Alliance says the government could save £888,000 (almost $1.8 million) by purchasing 1.4L gas engine Ford Focuses. The price differential would be enough to buy over 74,000 trees which would absorb 54 million kg of CO2. TPA analyst Matthew Sinclair says you would have to drive a hybrid six million miles to save that much CO2.
As so often happens, statistics can be manipulated to support almost any side of an argument. Although a single hybrid would indeed have to go six million miles, the cost savings were based on 110 hybrids which amounts to 54,500 miles per car. It's not clear if the 54 million kg of CO2 absorption is total or annual, but either way the argument is far less clear cut than the Taxpayer Alliance makes it seem.