mpg down since new plugs fitted

Hi all
i have a 2000 p38 4.0 hse running on zavoli lpg.
i had a problem with the maf so replaced with a second hand genuine one (heard alot of horror stories of the new cheap ones on ebay not workin)
and as soon as i changed it i noticed a big difference in the way she drove,much better,was actually worth putting into sport mode
but i decided to change the plugs aswell yesterday and everything went fine,apart from the obvious grazed knuckles etc,started her up and as usual started straight away but i noticed my mpg dropped from about 12.5 to 8.5 mpg,now i know computer isnt always right when it comes to mpg but cant understand why it would drop so much just after having new plugs.i used standard champion copper plugs as was advised the expensive ones are a waste of time.
any ideas gratefully received

Did it jump from 12 to 8 or did you reset the computer when you changed the plugs? If the latter I'm assuming a cold start so it would be running rich so about 8mpg would be normal. You probably need a good run and to get it up to temp to see real life figures.

Yea,hadn't thought of that but it was cold,doing bits on it all day then drove it for short journey and reset computer.
Thanks for quick answer,I'll keep an eye on it : )

If you run the engine with the car stationary for any length of time the comp reading will drop like a stone. You are using the fuel but not doing the miles, so readings are based on mileage since last reset.

Thanks wammers,only had it running for couple of mins stationary.