I am missing what your gas mileage is, and the specs on the car. When I had a car with the electronic fuel economy thing, I noticed my gas mileage was suddenly not nearly as impressive when going down a steep hill - then it was wheel bearings, which for some reason, this time did not make the characteristic squealing sound they normally would make when bad - just had greatly increased rolling resistance. Also noticed at one point that my car engine would run COLD when going down a steep grade - thermostat was bad, not closing off again when less cooling was needed. Maybe drive with windows down at low speed, do you hear anything? Scotty Kilmer has video's also on how he uses wired microphones for car diagnosis, sits in passenger seat while someone drives, and has these microphones placed around various parts under the car, so he can hear from different parts when driving - helps to pinpoint sounds -