Gas mileage

I have a 2003 Buick Rendezvous (SUV) with 87,000 miles that used to get 25 city and 28-30 Hway. The last 3 yrs, I’m down to 22 city & 25 Hway. What can I do? Oil gets changed every 4,000; fuel & engine air filters replaced + fuel injectors flushed at 60,000 miles. Miss the old mileage.

Change the thermostat.
Check for dragging brakes.
Check wheel alignment.
Test compression.
Test front O2 sensor.
Clean the MAF sensor.

What tires do you have on now, and what tires were on it originally? What is your tire pressure, and how often do you check it? How have your driving habits changed? Do you drive exactly the same roads under the same conditions? Are the roads more congested now?

When you consider that your car is rated for 16-17 MPG city and 22-24 MPG. I don’t think you have any reason for concern. You’re beating the official numbers by a wide margin in the city and slightly beating them on the highway. I doubt if there’s anything wrong with your car.

"What tires do you have on now, and what tires were on it originally?"
This is an excellent question posed by jtsanders. You report that you now have 87,000 miles. Thinking as a detective, I’ll bet you purchased new tires about 3 years ago and this was the time your reported gasoline mileage declined. Check the size of the tires and then check the placard on the left driver’s door pillar to make sure the same tires on the car are what the manufacturer originally specified.
If the tires are a larger size, you aren’t getting different mileage–your odometer is reading differently.

Have you changed fuel type? Are you now using regular in a car that specifies or recommends premium?

Another possibility, three years ago all the gas stations were selling 100% gasoline in your area and now they are all selling gas that’s 10% ethanol.