How does the MPG readout work when running on LPG?

I picked my car up from having the LPG fitted this morning and was surprised to see the MPG readout working. I expected it to not know what to display as it couldn't measure any petrol flow.

Does the LPG system feed back to the computer to allow it to work?

On the way to have the system fitted it was showing just under 21 mpg, on LPG on the way home it was averaging around 19 mpg. Given that the MPG was lower than it was on petrol I assumed this meant it was reading accurately for LPG.

The MPG computer only knows about petrol. If the LPG is adjusted perfectly the trims won't change between petrol and LPG so the average mpg on the computer won't change. That's not the same as measuring how much LPG is actually being used, because the LPG ECU will be squirting in gas according to it's own map. The LPG ECU isn't fully independent. It reads the petrol injection times and adds injector timing to allow the LPG injectors to flow the correct amount of gas.

The Petrol ECU is still in charge of petrol injector timings and will trim them according to the readings it is getting from the O2 sensors. If the LPG is set a little lean, the petrol ECU will compensate with longer injector timings to bring the O2 reading back to target. This will show up as higher fuel consumption on the dashboard but you're still only burning the correct amount of LPG.

Morat seems to have summed it up nicely. What you have to remember is that the computer doesn't measure petrol flow, in fact it doesn't measure anything. All it does is calculate it from what it can see. It knows what the fuel pressure is and is being told how long the injectors are being opened for so from those two readings, it can calculate how much fuel you are using and therefore your fuel consumption. If the reading when running on petrol and LPG are identical, then the LPG system is set up absolutely spot on and the petrol ECU isn't having to alter the fuel trims to get things correct. Run it around for a while to let things settle, then check it again. If it gives a wildly different reading when on gas to when on petrol then it will need the calibration tweaking but if it is roughly the same, then there's no need.

Cheers guys!

I'll have to do a proper fill up to fill up manual calculation to get an idea of the LPG MPG then.

Brim to brim, same pump, use the trip meter. The best measure is Miles/Litre which gives you a quick way to see cost per mile. Most people on here seem to break 3 miles/litre. I don't quite. Possibly because my wife regards the throttle as a switch and we live on back roads. 😁

I think I might code myself a nice little Android app for my phone to calculate my MPG, miles per litre and pence per mile :P

I might even have it compare the cost of LPG vs it what it would have cost me on petrol (based on the average mpg I got when running on petrol).

I assume you'll be back at work soon. Then you'll have Flogas just down the road so you can fill up at 54p a litre. I assume Simon fitted a 4 hole tank so the pipes can come out of the side of the wheel well and not out of the bottom (like mine) where they can get clouted when you go off roading?

I am going back to work soon but not in Peterborough. Dropping to 3 days per week and paying for nursery fees means the commute to Peterborough wouldn't be financially viable (even on LPG).

We're moving to Huncote (just outside Leicester), to a house owned by Fiona's parents who are giving us reduced rent :)

I'll then be commuting to our Coventry depot. Commute isn't much different to my commute from here to Peterborough but coupled with the rent saving it's worthwhile.

There's a Morrisons not far off my new commute that has LPG for around 55ppl.

I'm not sure what type he has fitted but the pipes exit through the bottom of the spare wheel well. I asked him to locate the filler closer to the towbar to reduce the chances of knocking it while off road but I forgot to mention the other pipes. I'll need to have a look to see if moving them is something I can do.

If the pipes exit through the centre of the wheel well like mine, then he's fitted a single hole toroidal tank which has the inlet and outlet in the centre. This is the standard install on most cars. As it has a gas tight lid and a vent out of the bottom, you can't take the pipes up and out the top. A 4 hole tank has the connections on the outside of the tank so could come out of the side of the wheel well. There's nothing to stop you putting a skid over the pipes to protect them though.

Skid plate was the backup plan :)

To be fair to Simon, I didn't mention anything about off road use and preferred pipe routing at the time we arranged the fitting.

I've got a "zero degree" single-hole tank, with one port sticking out the side where the multivalve is nicely protected from clouting by the rear axle :-)

What do the degrees mean?

From the paperwork I have it looks like I have a 30 degree 89l tank with an 80% fill valve.

I guess 89l is max capacity, 80% is what I'm limited to filling it to. Not sure about the degrees though.

I think it's the angle the multivalve neck sits at.