Could fuel economy has dropped be caused by dirty air filter?

Restricted intake = poor combustion = worse mileage.

Could fuel economy has dropped be caused by worn spark plugs?

Weak spark wastes fuel. Plugs over 60k miles are a common culprit.

Bad O2 readings throw off fuel mixture. Often costs 10-20% in MPG.

A dragging brake fights every mile you drive. Check for hot wheels after a drive.

Each 1 PSI low costs about 0.2% in MPG. Check pressures monthly.

Severity is rated Low. Check tire pressures first (free), then air filter. If those are fine, scan for codes and consider O2 sensor age.

A sudden drop in MPG always has a cause — usually engine efficiency or a sticky brake.

Severity: Low

Check tire pressures first (free), then air filter. If those are fine, scan for codes and consider O2 sensor age.

Dirty air filter: Restricted intake = poor combustion = worse mileage. Shop air filter
Worn spark plugs: Weak spark wastes fuel. Plugs over 60k miles are a common culprit. Shop spark plugs
Failing oxygen sensor: Bad O2 readings throw off fuel mixture. Often costs 10-20% in MPG. Shop oxygen sensor
Stuck brake caliper: A dragging brake fights every mile you drive. Check for hot wheels after a drive. Shop brake calipers
Underinflated tires: Each 1 PSI low costs about 0.2% in MPG. Check pressures monthly. Shop tire pressure gauge