How much Hydrogen to match Petrol

[Hydro-Cell]

well if your planning on using HHO then you will need about 630lpm to run the same car.

HHO in my personal opinion is a bit of a waste of time.
fair enough it has its own oxygen source so no outside oxygen is required, but your creating more work for yourself.
the Oxygen is free its in the air, plants do a brillinat job of providing it for us, why go through all the hassle of making your own. i prefer to use pure H2. that way i can choose what ratio i burn it at.

[Cloxxki]

well if your planning on using HHO then you will need about 630lpm to run the same car.

HHO in my personal opinion is a bit of a waste of time.
fair enough it has its own oxygen source so no outside oxygen is required, but your creating more work for yourself.
the Oxygen is free its in the air, plants do a brillinat job of providing it for us, why go through all the hassle of making your own. i prefer to use pure H2. that way i can choose what ratio i burn it at.

Where would the H come from then, and what would it cost, to produce 420 litres of it?

[Hydro-Cell]

to produce 420 litres it would cost about £0.45p

[MasterPlaster]

You are not comparing like with like. A few days ago I came across a description of how high mileage carburators work.

They work on the pricipal of exploding the same fuel rather than burning it
and thus manifesting different dynamics. I am only repeating this and personally have not experimented with this field.

http://www.himacresearch.com/books/secret1.html

[Cloxxki]

to produce 420 litres it would cost about £0.45p

For under 2km (a minute's worth). That will not get very popular even among the greenest of people :-)