You could have 100 of the exact same engines running the exact same route on the exact same fuel, all on the same day, and have 100 different MPG numbers. Just small differences in machining can and will make a big difference.

The biggest differences would be between the last engine cut on tooling at the very end of its life, vs. the 1st engine cut on fresh tooling. Big power differences between those two as well if you were to put them on a dyno. This is why custom tunes are always MUCH better than buying a canned tune that works on every vehicle. It'll be close, but those minor differences between the same engine come into play.

 

Mine is pretty bad ngl.

 

1999 Envoy, 3.73's 340,000 Km = 211,000 miles.
Best milage is taking it easy at 80KM = 10.9 /100km or 50 MPH = 21 US mpg
Normal Hwy driving @ 100 - 110 Km = 16.4/100 or 60 - 65 MPH = 14.3 US mpg
In town milage average is 17.1/100 Km or 13.8 US mpg
Envoy is good condtion, still doesnt use oil between oil changes and oil stays clean.
Used to tow 25 foot boat about 5 Km to launch... About only time 3.73's were useful.
She handled the boat weight no problem. Used 4X4 to keep load down on wimpy G50 rear differental on ramp.

 

I have a 1997 5 speed and I don't drive hard, currently 140k miles. Replaced almost the entire fuel system except the lines and tank. Installed new timing chain, valves, lifters, push rods; oil gets changed every 3k miles; Differential, transfer case and transmission fluids have been maintained. Good brakes, rotors, drums, wheel bearings and tires. New radiator, hoses and water pump. I'm averaging 16-17 mpg. Used to be closer to 19-20mpg. Age on the motor is catching up I guess. No signs of compression loss or worn rings, but couldn't hurt to replace those. I'll do that eventually once I have my own place and equipment to do that. Probably easier just to get a new crate motor.

 

If the station you're using started adding more ethanol to the fuel, that will kill your mileage. So will a failing o2 sensor, old plugs, wires, cap, rotor, a dirty air filter, fuel filter, low tire pressure, among many other variables, like driving it harder.

I've found that most people will drive easier when they first get a vehicle, and that'll buy you higher MPG's. Then as they get used to the vehicle they'll tend to jump on the throttle more, corner harder, etc..

Did it myself with my '07 Silverado 1500 4x4 with the 5.3 I bought brand new & owned 16 years. Started off at 18 MPG, then to 16, and by year 5 it was around 14.5 where it stayed until I sold it. After all the years of owning it, it felt like an extension of myself. I could push it right to every limit.

 

13-14mpg. 2002 Blazer LS 4x4. 3.42 with G80. P235/75R15 Wrangler Trail Runner A/T's. Mix of city and freeway driving. No cat, full tune up, aluminum dizzy, stock poppet spider. This mileage whether I'm nice to her or not.