I have a 2004 Chevrolet Tahoe 2WD with the 5.3L V8. Tahoe has 134K. At about 125K miles, I replaced air filter with a K&N, new OEM spark plugs and spark plug wires. The Tahoe has been running great and was getting normal mpg until recently. 15mpg in town and 17mpg on the road is what I normally get. The last 3 fill ups my mpg has been below 13.5mpg. Car still runs great but at 13mpg it’s starting to hurt a little financially. Weather here in NC has been bnormal, my driving conditions or habits have not changed, etc. What could be causing my mpg to drop by so much while the suv still rides smoothe and straight?
Check your tire pressure. Use the placard on the vehicle to check for proper pressure. Low tire pressure (even by a few pounds) can impact fuel economy. How are you checking your fuel mileage?
A bad coolant temp sensor will cause the engine to run rich and reduce mpg. A stuck open thermostat will make the engine run cold and also reduce mpg. Check those.
While others may disagree with me . . .
Those aftermarket air filters have been known to damage the mass airflow sensor, on occasion
A malfunctioning MAF could affect fuel economy and other things
I suggest your bring your car to a competent shop, and this is why
Check for any stored and current fault codes
Check the fuel trims
Check if anything obvious is faulty
And I agree with the suggestions that @missileman and @jesmed made
I agree with @db4690. I never brought up the K&N filter because I have ridden that horse for too long. When I had access to a dynamometer…I proved many times that this filter did not improve fuel economy or power. A high quality air filter got the exact same results as a K&N filter and leaving the filter off got the best results. Leaving the filter off is a bad idea so I always advise anyone to just buy a quality filter and forget the hype.