I was not aware that the 06 HAH's gas mileage is less than the 05. Honda told me the upgrade is for the pre April 06 HAH. Our 06 HAH was built 11/05. The IRS increased the tax incentive for the 06 HAH with the engine control upgrade which Honda it will notify owners of the pre April 06 HAH to go to their dealer in late July for the upgrade. I called Honda in L.A. They were very helpful.