CO 2 emissions

Hi , can anyone clear up my problem , I have ordered an new A3 sportback , 1.8 tfsi s/tronic black edition
the dealer says its £175 road tax for 157 g/km .
the audi brochure and website say the emmisions are 149 g/km = £125 road tax
other sites on the web all vary , I have e-mailed audi uk and not yet had a response
my salesman in on hols this week so i cannot speak to him et .
Any ideas where i can find the true figures I dont want to spend £50 more than i have to on road tax

thanks

many thanks for the link which has confused me even more , they say 154 g/km which is £155 road tax after april this year ., SO why are AUDI printing the wrong figures in there brochures and on there website ?
I will ask them to pay the other£30 as i was expecting £125 road tax per year . After all it is misperesentation of the facts .