Since many people are claiming big improvements with their HHO systems, but there were no verifiable hard data posted here, I'm giving all HHO owners a 'can't refuse' offer:
I'm willing to pay for the gasoline, plus a reasonable amount for your time ($100 or $12/hour, whichever is higher) to have your mpg tested.
Where's the catch? I will be there and do the fill-ups, but let you do the driving. The drive must be at least 150 miles one way, 300 miles total, should take place off-peak, and using cruise control when the conditions are good. There must be an easy way to disconnect and reconnect the electrolyzer.
I strongly prefer local 'driving' (i.e. Seattle, WA to Portland, OR, and back), but other arrangements are possible.