Hi I have a jazz retro fitted with LPG ,so have a petrol gauge and a separate gauge for the LPG. When I drive on the LPG my petrol gauge goes down like I am using petrol. When the petrol gauge gets down to zero it has a ducky fit then goes up to correct postision. The LPG and petrol gauge are not linked in any way at all. I think the computer must work out what petrol you have in from your mpg ? In two months driving have use about 2 gallons of petrol as it only runs on petrol for about the first 10 mins till it hits the correct temp.

 

I'm thinking your Fit is not USDM? Is the Bi-fuel option from the factory or a conversion?
Here's a link to a UK forum for conversions(if you haven't been there already)
http://www.lpgforum.co.uk/viewtopic.php?f=28&t=5131