Depending how i drive my milage can vary from 11 to 19.
But it sounds like this is a change from the norm. How long have you had the blazer? What temp T-stat do you have in it?
Where are you located? What are the weather conditions? If its africa hot where you are and youre running the AC, chances are the fan clutch is locked in most of the time. That bugger can move some serious air!
You might also have a lazy O2 sensor.
Just tossing ideas.

 

I'm averaging 13.5MPG on my truck, although we have completely different setups...

 

In the winter is when mine gets an average of 13-15, but the other seasons I get 19-22. My dads gets 18-20, and my mom doesnt check hers cuz she uses hers on a mail route so itd be terrible Im sure. Mine is a 97, and theres are 99s.

 

I've got a '99 and an '01. I get about 13 mpg with both. Great trucks, sucky mileage. Just deal with it.

 

2k 4x4 4d
before ecu tune 15/17 70-75mph interstate
after tune 18/18 70-75mph interstate also
now I getting 65mpg with bike so when I do drive I have fun with it so don't even want to know what I getting now.

 

My rear calipers (04 4x4 LS) were sticking and dragging the brakes all the time. Was getting around 13 mpg on mostly short trips. After replacing the rear calipers, it drives much better and looks like I should get maybe 40+ more miles per tank.

Just something to consider, maybe yours is doing the same?

 

Normal conditions, wife's drive to work, 12 mi each way.. avg mpg is 15

I did 21.3 on a 360 mi round trip, almost all interstate Cruise set @65mph.
You guys with tripminders set cruise at 70 and watch inst mpg on the flats. Then set it at 65 and check it.

I was astounded. almost 4 mpg difference. That's 20% if you dont get it otherwise

 

I get 11.5 mpg with my 2004 4dr/4wd/ auto. It stinks on fuel. One thing I did notice with this Blazer is the declutching fan. I hear it all the time when I pull out from a stop. I can't say my 2001 or my 1997 ever did that after it was warmed up.

 

Man I know that my owners manual says I have a 19 gallon tank, but when I fill up at the pump It says only 12.5 gallons just a suggestion or even just a thought but were are you getting the number your dividing from. Are you getting it from a book or on the amount your putting in it????

 

I take the miles driven on the trip odometer and divide by the gallons to fill.

ex: 173.3 miles driven, divided by the 15.74 gallons to fill = 11.01