Well . . . larger tires, is about the same as lower the rear diff ratio, so IF he runs lots of highway, I could see a gain in mpg.

I agree driving style is probably the biggest factor . . . when I was driving OTR (Over The Road) semi trucks, started back in 92, industry norms were about 5 mpg running 80,000 pounds around the country, the company I worked for offered a $0.01 bonus if we broke 6mpg! I consisantly got the fuel bonus and most of the drivers couldn't because they always had the foot on the floor (we ran 325 hp gov @65 mph) patience, planning ahead, and only idling overnight if it was extreme temps, as we got newer trucks an d more powerful engines (350 hp!) I was always over 7mph, and hit as high as 11 mph one trip! (mostly level running, and I was not in a hurry and kept it at 55 mph with a light load, no idle at night)

I bought a ScanGaugeII for my gmc pu, it plugs into the OBDII port, and gives you instant read outs, it "Teaches You To Drive" effeciently! same as any car with a instant mpg gauge, watch that and you will see you are letting off the go pedal as you go up the hill to keep the mpg high!

Back in the late 80's I had a 81 Caddy Deville w/4100 motor 8cyl, Normally on my way to work in the next town over I had to go over this one hill about 1 mile from where I lived . . . I noticed that if I maintained 50 mph (the speed limit) I would drop to about 11 mpg! One day I had to come back into my home town in the middle of the day, and then go back to work, once the car was warmed up I climbed that same hill going the same direction keeping it at the same speed I got 18 mpg! (warm engine, tranny, wheel bearings . . .)