FORD ESCORT WAGON 1993 SUDDEN MILEAGE DROP?

134,000miles. Owned from new and babied. All services and 3000 oil changes - always got 30 plus until sudden drop to 20.Ford shop did exhaustive check for 2 days; replaced ECT and decarb intake valves, fuel injection flush, clean injectors,throttle body,mini-tune replace platinum plugs, MAF sensor and PCV hose. Car running like a dream - STILL getting 20 mpg!!! Any help please????