I'm trying to work out the CO2 emmissions for my LPG converted vehicle,
but am getting a very high figure .
I think I must be doing something wrong as it is in the region of 4
times the quoted emmissions for a BMW X5 4.8
I have seen figures of a 15% reduction in CO2 emissions so was
expecting something around the 275gpkm mark.

Would someone please check my calculations and advise where I'm going
wrong?

Assumptions:-
85%/15% Propane/Butane mix for Autogas
Specific Gravity of Propane = 0.508
Specific Gravity of Butane = 0.573
Avagadro's Number = 6.0221415 x 10^23
Combustion is 100%


Therefore 1 litre of Autogas contains 431.8g of propane & 85.95g of
butane

Therefore contains:-
(431.8/44)* 6.0221415 × 10^23 moles of Propane
(85.95/58)* 6.0221415 × 10^23 moles of Butane

As Propane contains 3 carbon atoms & Butane 4 atoms of carbon:-
177.29732043409090909090909090909 x 10^23 x 3 atoms of carbon from
Propane
35.696762891379310344827586206897 x 10^23 x 4 atoms of Carbon from
Butane

Total 212.99408332547021943573667711509 x 10^23 atoms of Carbon

When this is burned, it will produce 212.99408332547021943573667711509
Moles of Carbon Dioxide
Multiply this by 44 (Molecular weight of CO2) =
9371.73966632068965517241379306 grammes of CO2 per Litre of Autogas
burned

Assuming an economy of 4.5 miles/Litre = 7.2 km/litre
= 1301.630509211206896551724137925 grammes of CO2 per km

Quoted BMW x5 4.8 emissions are 324grammes/km