Your 20mpg equates to about 4.5miles/litre
I have a manual S40 1.8 2002 bifuel with 123K on it. Car has a Necam (Koltec) EGI factory fitted system in it. It also has slowly declining LPG economy. It's declined in 2 perceivable 'steps'. From new and maybe upto about 80K miles it would do 250-270 miles per 42L of LPG ( 6.5miles/litre) on the motorway and 180-220 miles per 42L ( 4.3 to 5.2L/mile) Urban. The most it ever did motorway was 273 miles on 42L. Air con on this car has never been switched off. I can't tell you the petrol mileage as I've never put more than £10 of petrol in it at once. The day I paid more than £20 to fill its LPG tank I needed counselling. 99.99% of its miles have been LPG. I'd guess I've never put more than £100 of petrol in it ever. I've only put that much in it due to fault '2' below.
Original spark plugs were Volvo 3 point ones. Volvo 3yr service history until 68K. After that SFE65PRS-GAS plugs from
www.lpg-kits.org gapped at 0.6mm and I haven't noticed a difference. They've been fine. Mind you unlike some on here the Volvo 3 pin ones were fine to. Personally I don't reckon there's any difference between 3 and single pin spark plugs as the spark will always arc to the easiest path. As one path erodes down so it will pick another pin. My guess it that perceived differences have been caused by other subtle factors.
I concur with the people on here that in hindsight, and I used to question them at the time when the car had distributor problems, that the Volvo main dealer knew little about the LPG system. They could answer some questions but beyond a certain point they seemed lost. For the first 50K miles or so it would sometimes stall at changeover, or braking for roundabouts etc. The fault seemed ambient temperature related. Cold mornings were worse. Sometimes you could get it to adopt LPG by running on petrol till warm then switching over. Volvo's worked seemed to consist of cleaning the residue out of the LPG distributor. At one point I heard talk of an upgrade being fitted (but to what I never got out of them) and after that the problems were gone. Its now done about 50-60K miles largely untouched.
Two things have happened since about 80K miles.
1) The tank won't take 42L anymore. The most is has ever taken on a bitterly cold morning was just over 43L. Now it only takes 37-38 Litres to fill it. About 10% of its capacity has gone. Is this a build up of 'goo' in the tank?
2) The car consumes more petrol now but the driving style is unchanged and the changeover to LPG happens just as quick. It 'looks' like it's burning both petrol and LPG at the same time, as the petrol is still consumed more than I would expect on long trips. Is this possible? Fuel economy on LPG on the motorway now is about 190miles per 38L. So 5miles/litre. Even allowing for the reduced tank capacity that would be 210miles at 5miles/litre. Urban it is now 150-185miles on 38L. So about 4 to 4.9 miles/litre. Interesting that the difference has disappeared.
Currently under investigation. Vapouriser pressures are correct and the diaphram doesn't leak. Distributor has been cleaned and redone with new seals etc. LPG Vapour side filter flushed out. This made it run more smoothly on LPG but didn't effect the fuel economy, with one possibly anomaly. When the distributor was completely overhauled, the first tank full of LPG I could swear I got much better mileage out of but it never repeated after that. The battery had been disconnected at the same time so I was never sure what, if anything, had caused it.
If a sensor was down somewhere and the LPG ECU was in 'limp home mode' would this explain the falling gas mileage?