[teoman]

Sounds plausible, but if it were really %30 as you state the my vapor burning engine (ml350 3.7L converted to lpg, runs on heated lpg vapor) would get much much better mpg.

So there is probably more to it.

So it could be an engine tuning issue that i do not see the mpg increase. But thenagain cars made specifically for lpg do not exhibit such remarkable mpg improvements either so there must be more to it.

Don't let me discourage you. You can get some mpg gains. If not you will end up with a system that can conver used motor oil and waste vegetable oil to fuel on the fly...