Surely filling it with helium would make it lighter and give more mpg 
done that for sh!ts a giggels.
helium expensive..... helium also very small atom and its very good at diffuision through tyers.
sods law also states that youd hear a lot of squeekey voices and find your tyers flat all the time if you ever told anybody.
in theory pressure would be less affected by tempriture as well
