Ford will replace ignition switches on up to 8.7 million 1988-'93 model vehicles in the U.S. after a National Highway Traffic Safety Administration investigation into 816 reports of fires caused by a short circuit. It is NHTSA's largest recall applying to one automaker, surpassing General Motors Corp.'s 1971 reall to repair faulty engine mounts on 6.7 million vehicles. At a repair cost of between $50 to $90 per vehicle, Ford's exposure could run as high as $765 million. Typically, about 68% of notified owners respond to safety recalls, NHTSA says. The specter of fire -- about one-third of the fires occurred when the vehicles were parked -- could generate a higher response in this case. Ford has heard of 21 injuries related to the fires in the U.S. and nine in Canada. The company already has recalled 248,000 vehicles in Canada for the same problem, and agrees to notify an additional 611,000 owners there. The affected models include Escort, Mustang, Tempo, Topaz, Thunderbird, Cougar, Crown Victoria, Grand Marquis, Lincoln Town Car, Aerostar, Bronco and F-series trucks. United Technologies Automotive, the ignition-system supplier, says a new design, introduced in late 1992, eliminates the risk of short-circuiting.