My brother'* car. He'* only getting ~15 MPG.

Within the past 1,000 miles we replaced the following:
-- LIM gaskets
-- UIM
-- UIM gaskets
-- PCV
-- Delco wires
-- Delco plugs
-- Delco O2
-- 195 thermostat
-- Coolant flush
-- Air filter
-- Fuel filter
-- Tranny fluid and filter

Car has 123K. His gas mileage was really bad before we did all the work, too. I figured his thermostat was stuck open (or there was a 160 in there). Neither were true. So, we did the plugs, wires, new thrmostat anyway, and an O2. The gas mileage is still really crappy.

He typically lets it warm up 5 minutes or so before driving it. And he doesn't drive it hard (he does mostly highway driving anyway).

Any thoughts? BTW, the cat does not rattle at all.

I'm sure you covered this....you're positive he did an accurate mileage check? Fill up to full then divide by how many...blah, blah, blah..

I would assume winter gas is still being used so that'* going to knock off some mpg.

Are the tires properly inflated? Any signs of a dragging brake/*?

Transmission working properly?

Trans is working beautifully.

Tires are properly inflated at 32 PSI.

Didn't check the brakes. That'* a good one. We'll jack it up and make sure al the wheels free spin.

I know he'* calculating correctly. Fill it up, reset the trip odo, when it gets to about 220 miles, he'* about on E, fill it up, record the gallons, divide the exact gallons into the exact trip odo reading.

That 5 minutes of time is enough to make a noticeable difference in your MPG.

Tell him on the next tank of gas, get in start it, let it run for 20-30 seconds, and then go. Keep the RPM'* below 2500 until the temp gauge starts to move. Once that gauge is moving, the oil is MORE than warm enough to move around freely. See how much of a difference it makes.