I know some cars the gas gauge can be out of calibration on my 88 Accord when I fill it up it stops at 3/4 tank sometimes it goes all the way up but will drop quickly to the 3/4th mark also what mods do you have and do you have a lead foot as far as the lines are concerned I have to look under my car to see where exactly its coming frombut they are high pressure tubes if its coming from the back it could have something to do with the gas lines so that could be the very reason your gas mileage is poor hope this helps.