Brake manufacturers are wary of the National Highway Traffic Safety Admin.'s plans for a vehicle rating system based on stopping distance. NHTSA is expected to unveil the details of the testing program by the end of June. Concerns center around variation, says Philip Headley, chief engineer for Continental Teves GmbH, which, May 1, marked production of its 50 millionth electronic brake system. “You can buy the same car with three different tires, three different tire sizes. And that has a big effect on stopping distance,” Mr. Headley says. “They can't possibly test all the combinations, I don't think.” He says testing surface, weather conditions and differences in testing methodology for ABS and non-ABS equipped vehicles are other areas of concern.