OK, I've run a "real world" test of magnets on the fuel line and air intake.
Three fills without magnets -
2169 km, 112.81 litres =
5.66 l/100km average.
Three fills with low powered ferrite magnets -
2425 km, 137.72 litres =
5.68 l/100km average.
Three fills with high powered rare earth magnets -
2118 km, 119.79 litres =
5.66 l/100km average.
This wasn't a controlled test, just me driving to work as normal (winter), but I think the results speak for themselves.
Pete.