ADVICE NEEDED ON LPG MPG

Message

davetheclutch · #1 Post by **davetheclutch** » Sat Feb 05, 2005 5:42 pm

I just had an LPG conversion done on my 4.0l jeep 1998 Grand Cherokee. I had a full tank of gas (56litres) and I have just come back from the coast for the first time on using it. I travelled 120 miles before the gas run out. I done a few calcs and it works out at 9.69 Miles per Gallon based on £1.81 a gallon that I paid. On the overhead CPU I was told I was travelling 22.56 MPG. I am going to ring the installer Monday to find out why but can anyone shed some light on it in the meantime. I calced the total cost of petrol over the same trip at 22.56MPG and it works out cheaper!!!. Surely this cannot be right. Any help would be appreciated. Thanks

davetheclutch · #2 Post by **davetheclutch** » Sat Feb 05, 2005 5:43 pm

by the way, I calculate I should be getting about 223 miles out of a tank of £22.50 based on 18MPG. I got 120 lol

andyl · #4 Post by **andyl** » Sun Feb 06, 2005 2:02 pm

Did it actually use 56 litres, or did the gauge tell you it was out? The gauges are a little hit and miss. More miss and near miss actually. Don't know where you live, but 1.85 (41p/ltr) is expensive, I usually pay 36, but can get it for 34 or even 29 in some places. Your petrol mpg shouldn't differ by that much from the gas mpg, but gauges and pumps all vary tremendously. My mpg at different times varies from 10 to 20 mpg, depending on how accurate the gauge is that day and where the pump cuts off. It averages out at 14, whereas I used to get 16.1 on petrol, so I guess that's about right. Andy

davetheclutch · #5 Post by **davetheclutch** » Wed Feb 09, 2005 5:57 pm

Thanks for the response. As a test I have drive 60 miles on petrol under the same conditions as for gas. I have calculated that the jeep done 19.2 miles per gallon, as opposed to 10.9 mpg on the gas!!!. This difference is way too much surely. I know there is a drop but that is ridiculous. Can anyone explain why there is such a big difference. It is no use to me at that economy. All conditions were the same as before so I am baffled.

lenmorris · #6 Post by **lenmorris** » Wed Feb 09, 2005 10:51 pm

Dear David, The matter is quite simple. For any given performance the car needs a given amount of energy and it's not bothered where it gets this energy from. If it's petrol then let's say it will use a gallon. If it's LPG then it will be 1.33 gallons for the same amount of energy. It's all about energy density. On a weight basis LPG is better 46 MJ/kg compared with 42 MJ/kg for petrol. On a volume basis LPG looses out a little as petrol is about 0.75 kg/litre and LPG is 0.512 kg/litre. If you play with the figures to get a clear understanding of the volume of fuel required to deliver the same energy you'll see that your mpg on lpg should be exactly 25% less than your mpg on petrol. This is your base line indication that your car is tuned correctly. Many people will claim differently. This is normally because they are comparing a badly tuned or worn petrol system with a new lpg system. If you are getting 19.2 mpg on petrol then you should be getting 14.4 mpg on lpg. If you are not then something is wrong. It sounds as if you are running far too rich under load. Easy to check with a portable CO meter on the road or on a rolling road. Regards, Len Morris Lpg Auto Power UK Accrington