LPG MPG is likely to be the same as petrol if not a little less due to the lower calorific content of LPG over petrol...its the price which makes it appear to offer better consumption.
Besides LPG is normally a conversion, so the trip computer or display doesnt know what consumption your getting from the lpg itself...only the petrol. It will appear low because normally you only use petrol on start up when cold. Lpg then kicks in automatically when you go over 2-3000 revs.
so if you were getting 15 mpg with petrol, you'll get about 15 mpg with lpg, but for half the price, which in effect makes the equivalent of about 30 mpg if you were driving petrol
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