Reminds me of when they updated the SAE standards for determining HP ratings of combustion engines, about 15 years ago, to standardize some of the testing conditions, such as temperature, etc... Some Honda engines ended up with lower HP ratings at the time. For example, the Honda J32 in the Acura TL used to be rated at 270hp when the engine came out under the old standard, but got de-rated to 258hp after the revised standard was used. Honda got a lot of flack, because Nissan's VQ35DE at the time, maintained its 280hp rating even under the revised testing standardsBeyond the high level overviews at the above, due to a Hyundai MPG class action lawsuit settlement, they put out some other info that they felt could lead to variable results. Examples of that below:
Modernizing the Fuel Economy Testing Process - Hyundai MPG Info
Fuel economy is measured under controlled conditions in a laboratory, using a set of tests that are set forth in regulations issued by the U.S. Environmental Protection Agency (EPA). The test results are used for two purposes: (1) to establish compliance with the fuel economy and greenhouse gas...hyundaimpginfo.comBACKGROUNDER: U.S. EPA Fuel Economy Testing - Hyundai MPG Info
Current EPA requirements for fuel economy testing are inexact in that they provide vehicle manufacturers with wide latitude. As a result, vehicle manufacturer test results as a whole are imprecise, inconsistent and not repeatable, as noted by consumer advocates ranging from Consumer Reports to...hyundaimpginfo.comResource Center - Hyundai MPG Info
Fact Sheets and Backgrounders: FACT SHEET: Hyundai Settlement with the U.S. EPA BACKGROUNDER: U.S. EPA Fuel Economy Testing FACT SHEET: Coastdown Facts BACKGROUNDER: Greenhouse Gas Emissions Standard and Credits BACKGROUNDER: Fuel Economy Ratings Further reading about the current method through...hyundaimpginfo.com
Automakers can (and likely do) push the boundaries in order to receive better results while still staying with the rules. How Fuel Economy Is Measured (And Why You Get Different MPGs) makes a reference to the above dispute between Hyundai and the EPA.