couldn't there be a device which accurately measures the weight of the bladder? that seems to me to be an easy way to get an accurate fuel gauge...
Yeah... my father had a '61 Beetle years ago that worked like that. From what I hear, the bummer is that you had to fill up both the tank and reserve separately, so if your primary runs out of gas and you switch to your reserve - but did not remember to re-fill it after the last time - then you will be SOL... I think we (society) tend to take for granted the simple luxuries like a fairly accurate fuel gauge on modern vehicles. - Kevin