Do you let it warm up a lot before you get in and go?

 

Oxygen sensors are shot??

 

O2 sensor issues should cause a check engine light.

 

Especially if it's that much of a change, that suddenly...

It is possible to have an O2 sensor that's lazy enough to affect mileage and not trip a code... But not that big of a change.

 

Normally the MAP will toss a code, but I have seen some OBD II systems where it doesn't. This is one of the main parts I have seen cause that Large of a drop in MPG. Most of the others have been listed here already.

I would also look inside the plentum and see if the FPR is leaking (not that hard to get in and view that puppy).

 

Okay lets see, it has had a tune up and I calculated the gas mileage the way i have for years, you put gas in and then drive and see how many miles you go when you have to put gas in again and see how many gallons you used. The oxygen sensors were replaced already.The tire pressure is fine and I though of the 70s and 80s and did get a locking gas cap.It isn't a 4wd and the rpms at 70 miles an hour i will have to check next time I go out, what should it read?

 

My blazer is a ZR2 so I don't think my RPMs will be quite the same for you but around 70 mph= about 2100 RPM.

 

Sorry I haven't gotten back to this. I do not know at this time the rpm at 70mph. I know at 50mph it is 1500 but havent taken it out where I can go that fast yet as not driving it unless I have had too right now, with gas 3.10 a gallon I just havent done so yet but I will and will post it here as I still would like to find a solution.....thanks for all of you who posted help.....