It also suggests such an unclear taxation strategy discourages fleets from any widespread shift away from diesel and petrol-run vehicles.
The company says that in contrast to 20 years ago when petrol or diesel were the only options, fleet operators today are under pressure to consider a wide selection of environmentally-friendly fuels such as compressed natural gas (CNG) and liquefied petroleum gas (LPG).
'However,' the company said, 'the various tax advantages for these options continue to change periodically, leaving businesses totally confused about the financial viability of available fuel alternatives.'
Glass's chief commercial vehicle editor George Alexander said: 'Forecasting values for standard diesel-engined vehicles has proved to be hard enough, but how can any fleet operator predict wholelife costs for an alternative fuel vehicle when they know favourable taxes today could so easily be wiped out in the next budget?'
Alexander claims that what is needed if manufacturers are going to invest heavily in the development of new models that will attract confident buyers is a 'period of certainty for duty levels imposed for clean fuels – say six to 10 years'.