to compensate for the different rolling resistance of the dynamometer, not to produce an unrealistic load on the vehicle). Normally the quoted figures a manufacturer publishes have to be proved by the relevant authority witnessing vehicle/engine tests. Some jurisdictions independently test emissions of vehicles in service, and as a final measure can force a recall of all of a particular type of vehicle if the customer vehicles do not fulfill manufacturers' claims within reasonable limits. The expense and bad publicity from such a recall encourage manufacturers to publish realistic figures. The US Federal government retests 10–15% of models), to make sure that the manufacturer's tests are accurate. Real-world fuel consumption can vary greatly as they can be affected by many factors that have little to do with the vehicle. Driving conditions – weather, traffic, temperature; driving style – hard braking, jackrabbit starts, and speeding; road conditions – paved vs gravel, smooth vs potholes; and things like carrying excess weight, roof racks, and fuel quality can all combine to dramatically increase fuel consumption. Expecting to consistently perform in the face of so many variables is impossible as is the expectation for one set of numbers to encompass every driver and their personal circumstances. The ratings are meant to provide a comparison and are not a promise of actual performance. Concerns over EPA estimates For many years critics had claimed that EPA (U.S. Environmental Protection Agency) estimated fuel economy figures had been misleading. The primary arguments of the EPA detractors were focused on the lack of real-world testing, and the very limited scale (i.e., city or highway). Partly as a response to these criticisms, the EPA changed their fuel economy rating system in 2008 in an attempt to more adequately address these concerns. Instead of testing simply in two presumed modes, the testing now covers: Faster speeds and acceleration Air conditioner use Colder outside temperatures While the new EPA standards may represent an improvement, real-world user data may still be the best way to gather and collect accurate fuel economy information. As such the EPA has also set up a website where drivers can enter and track their own real-world fuel economy numbers. There are also a number of websites that attempt to track and report individual user fuel economy data through real-life driving. Sites or publications such as Consumer Reports, Edmunds.com, Consumer Guide, and TrueDelta.com offer this service and claim more