There are several layers of answers here are a few considerations:

1) Only worry about emissions from the vehicle (H2O from H2 and CO2/CO from Diesel.

2) Also consider the CO2 made as a byproduct of steam methane reforming

+ CH4+ H20 CO + 3H2 and CO + H20 CO2 + H2 and the emissions

from refining crude associated with producing diesel

3) There will also be significant H2 loss because H2 molecules are small enough to pass through just about any container - I usually assume about 4% loss of H2

4) Biomethane could also have significant loss of methane to the atmosphere. My best guess is about 1-2% but you could take precaution to reduce that or save some some investment cost by tolerating higher losses.

5) Differences in vehicle performance over time and maintenance requirements are also are also important considerations.

Run the engine installed with orsat appratus with diesel fuel first and get the data of emission such as Co2, Nox, water vapour. and then run with biofuel you synthesised and analyse for same data then by comparing both the data we can get the reduction in GHG emission and also note down exhaust gas temprature.

and the same time perforamce of engine also important to analyse for that analyse for break thermal efficiency and torque of engine by varying load and spcifc fuel consumption.

Consider this research article for exhaustive combustion, performance and emission analysis.

It will be helpful.

"Combustion investigation of ternary blend mixture of biodiesel/n-butanol/diesel: CI engine performance and emission control"