Carbon dioxide is produced by burning fuel and so carbon is a cost and reducing carbon emissions should save you money.  It takes about 395 litres of diesel to make a tonne of carbon dioxide and at £1.30/litre thats about £500/tonne.  So saving tonnes saves money - particularly if it can be achieved as part of your normal procurement cycle at no extra cost.  
The carbon dioxide you make from diesel is also one of the most expensive - a tonne of carbon dioxide from natural gas will cost £100, one fifth of the diesel tonne.  So next time you look at your organisation's carbon emission profile remember that not all carbon costs the same and that even if transport accounts for only 20% of your carbon emissions it may account for signficantly more of your carbon costs.