Miles/gallon: Newer vs Older Cars

The only comparison I have is with my old '77 W150 vs my 2009 RAM 1500. I keep track of all of the gallons of fuel and miles for my vehicles. The '77 had a 360 and a four speed with 3:55 gears and 31-10.50R15 tires The RAM has the 5.7 Hemi , an auto with 3:55 gears and 275-60R20 tires. The old W150 averaged 13.9 for 223,000 miles. The new RAM that came with a 15/19 mpg rating has managed to achieve 12.9 mpg for 51,000 miles. Good thing the new one has a computer metering the gas and deactivating cylinders! The RAM can pull a trailer equally well as the old W150 too. The RAM does ride a lot better and is a lot quieter with a lot of creature comforts though!