Do ya mean you cant completely fill em cos of the 80% shut off?You will get a 70 litre capacity, but the usable volume of that will be about 60 litres, since you can't completely empty them.
Milage wise, my V8 110 used to get about 11-12 mpg running around on gas, can't remember what the range was though.
My 4.6HSE has a single Donut Tank in the spare-wheel well (no idea of it's volume) but whenever I run out completely (as in car has no power and alarms to switch back to Petrol) the most you can ever refill to is 68-70 Litres on the pump. Range from full to empty is typically only 200 - 230 miles (if driven lightly(230) or harshly(200)).
The dashboard display says it's doing typically 15.4 MPG.
When I first started on LPG (a Merc 500SEL) LPG was 34.9p/Ltr.
Now LPG is 69.9p/Ltr it's getting less affordable by the day/government LOL.
thats about right mate. i could do 165ish if driven like a old git.twin 35 ltr tanks,filled mine up from empty and only managed 45 litres and that gives me about 140 miles,should i get more in
On my disco i used to have an 85 ltr tank which would hold around 55ltrs and i get around 110miles per fill up,cheapest gas around here was 58p altr.