[kaisen]

Almost any 2001-up 3/4 ton diesel truck will return within +/- 10% (2 miles/gallon) of each other if they are spec'd similarly.

Is it 2wd or 4wd?
Is it 3.55, 3.73. or 4.10 geared?
Is it a 5 speed, 6 speed manual? If automatic is it 4,5, or 6 speeds?
Is it dual rear wheels or single?
Is it regular cab, cab-and-a-half, or full four door crew cab?
Street tires or all-terrain? What size?

These things make more difference than what brand.

I had a client that had a 1997 Ford F250 7.3 Powerstroke 5spd manual 3.55 2wd reg cab and he regularly got 30mpg at 60mph.

My 1995 Dodge 2500 5.9 Cummins 4spd auto 4.10 4wd reg cab wouldn't do better than 20mpg (and I was pretty happy with that).