whats happening here
 

every time i fill up with lpg i clock the milage, on a recent trip to france (when the car was fully loaded). i recored 240 miles (motorway) to a full tank. back in blighty i am lucky if i get 160 miles combined, also i have noticed that after filling the tank , my fuel guage only reads 3/4 full. what could the reason be?
is there a sensor in the tank that could be the problem or is my local filling station ripping me off ?

v70 bi-fuel 2004

Didn't think you could fill it up 'full' so to speak, and gas can expand?

The poor millage could be any number of issues with fueling. Long distance motorway will give better MPG as the throttle should be steady most of the time. Is the lambda sensor healthy?

If you are not getting 80% into the tank then the shut off valve probably needs changing.
If you are getting 80% of the tank capacity in then chances are the float gauge needs a new float fitting

whats happening here
 

update- i have just filled up at a diff lpg station and recorded 48.5 lt, and fuel gauge is showing 100% full, at the previous lpg station i have been using for the last couple of months i never get more than 39lt and thats why my gauge is always 2/3 full.
so i am now lead to believe that my old lpg stations equipment is faulty and not my car.

and thats probably why my mpg is down too.

Often when you fill with LPG, especially if there is any water vapour in the gas, it will cause the sender to freeze so it can take a while for it to creep up as it warms. First time it happened on my camper van, I was convinced the garage had a faulty pump which had short changed me.