I have a 1991 ford ranger, 131,000 kilometers, 3L engine. I was getting 600km per tank of gas. In late Sept., I started getting 450km per tank. I have changed the spark plugs, pcv valve, gas filter, thermostat, air filter. Any other ideas?

 

Welcome to FTE candu1953,

Well...you should have your computer codes pulled so you don't end up playing "this part - that part". Some automotive stores will do this service for free, check around + save mucho denaro $$$!

or if your inclined, go to:
Pulling codes

 

Also test fuel pressure and check the vacuum line for the fuel pressure regulator for gas.