lets do some simple math here:
hydrogen contains 10KWH of energy per gallon...versus gasolines 36KWH per gallon (a ratio of about 3.5:1). so if we want to produce a 1 point increase in mileage (say, 20 MPG to 21 MPG..thats 5%), you would need to produce .17 gallons of hydrogen per mile.
since miles and/or kilometers per tank mean nothing, ill assume your base number of 620km per tank is at or around 20 miles per gallon. that means your new number, 775 km per tank is over 24 miles per gallon. for this increase, you would need to be producing almost 3 quarters of a gallon of hydrogen per mile.
that doesnt seem like that much of a stretch until you consider the fact that your using massive amounts of energy from your engine to produce this gas. the stock 90 amp alternator is capable of drawing 1.26 KWH in itself...so if we take this out of your 10 KWH per gallon of hydrogen number...we get .24 gallons of hydrogen to travel one mile, or almost a gallon of hydrogen per mile in your case.
not sure if any of this makes sense, but in short...you need to produce a gallon of hydrogen every minute at highway speeds to get your supposed 4MPG boost.