Many newer engines are just that much more efficient with the newer computers and technology.
My LS swapped 2wd blazer gets 22-24 MPG on the highway at 75MPH with 3:42 gears and 28in tires.
With my tune, my blazer cruises near stoich AFR and goes to around 12.5 or so when getting on it.
Many LS engines are set up to lean the mixture out when cruising to save fuel and go into power enrichment to enrich the mixture when more power is needed.

Also why many newer vehicles are going to direct injection engines. they can run a lot leaner thus saving fuel to get better mileage.

 

According to the computer readout on the roof console, I get 15.6, and that is entirely city driving.

The strange thing though is the instant economy, that’s all over the place, but usually reads between 20 and 50, which is impossible.

 

Report on 250 mile freeway and highway trip with little headwind or tailwind, if any at all.

First 150 at about 70 to 75 mph: 17.9 mpg
Second 100 at about 60 to 70 mph: 18.7 mpg

These values were what the trip computer reported. I then had to correct for the larger tires (30x9.5R15) vs. the stock ones (235/70R15) the truck computer assumes I still have. The difference is about 7%.

These numbers are more inline with what I remember from before I added the new tires.

 

Still strange

1994 all stock with 300'000 km on ODO (186'000 miles)
all serviced, tune up done some 5'000 miles ago

goes 17 - 19 mpg in mix, short trips and city
and 20 mpg when I use it on highway but don't want to save on fuel 120 km/h (80 mph)
and 21 mpg when I use it more to haul stuff with more highway use. Normal highway speed 100 km/h (65 mph)

All is down to about 14 - 15 mpg with a trailer.
No A/C thou - all R12 is gone and needs a rebuild.

 

I should have added that I have the 3.73:1 gears. I'd expect those with the higher gears would get better mileage.

 

I have a friend with a 5.7 L in a GMC Yukon SUV that got 19 mph Hwy. When that engine blew , he got a new GM crate 5.7 L put in it . Now he gets 17 mph Hwy? My trusty 2003 Blazer 2 door gets 17.3 mpg combined with AC on. .