From my experience at MCO and MIA, GM and certain Chrysler models (specifically the Pacifica) are connected with National's system to measure fuel. The receipts for those show "exact" gallon readings both in and out. At other airports they don't seem to be using it because the receipt shows normal gauge readings, 8/8, 7/8, etc. This is only for National; however over in the Avis forums there's a looong thread about many vehicles and connected gas sensors, so National certainly isn't alone in using these new systems. If anything they're not using them to the extent other companies are.