Never did this online before. 2003 toyota tacoma, 60K service performed at dealership, noticed drop in fuel mileage after service, pulling trailer with ATV mileage from 17.5 to 14.5? Suggestions???

their are a couple of reasons for that. the o2 sensor could cause it, faulty plugs, plug wires, cap or rotor as well as improper plug gap.

Thanks. I checked one plug and the gap is per the manual. Plug wires do not appear damaged and feel flexible. No cap and rotor, has coils. Did not check O2 sensor. Is there a procedure to check this? I also have been told maybe it is winter blend of fuel causing the drop in mileage. There is a rough idle that comes and goes once the engine is warm. Hard to describe, the tach does not move but I feel the rough idle. It comes and goes and was not there before the service.