My 1999 Blazer's fuel economy varies considerably with each tankful, despite driving the same driving conditions (highway speed, low traffic). My most recent tankful got me 17 m.p.g., the one prior to it got over 21 m.p.g. Is this a normal fluctuation, or is there a potential problem? It has gotten much colder recently (engine not properly warmed up?- my trip is about 15 miles each way). Anyone else have the same results?

 

Same fuel station each time?

 

To be honest, even your worst represented milage would be ideal for me.
i went from roughly 13 up to 15 after some tune up action and a CAI. then fell to 8 or 9 when my O2 crapped out and i melted my cats. Now I have bounced back up to 12.5-13, but can't seem to get back to where I would like to be... in the low 20's

there is a lot that factors into your MPGs. It's worth searching through.

 

My 97 Blazer does the same thing. Even if you buy from the same station they don't always get the same gas. Now winter blends are out. 17-21 mpg is good. Weather conditions play a part also. Remember these computer vehicles ajust to your driving & the weather, If one day you get up & the weather is a lot colder or hotter your computer doesn't adjust immediately. That hurt mileage etc. Just to give you an example. We have been having normal winter weather highs in the 30-40s lows teens & 20s. Well last week we got over a foot of snow & then the temps went to -25 below zero. with the highs in single digits. My wife commutes 120 miles a day. She took off at 5:30am with temps around -25 degrees below zero. She got about 10 miles from home & called me to tell me the check engine light was on. Well I told her to drive it to work, then call me. She did & I had her check a couple of things ALL GOOD. She came home that night I checked it, & cleared the codes & it has never come back on. The shop that does my work said it most likely was the computer calling for more gas because of the extreme cold. Our Jeep acted up also now that the weather is back to normal they both are fine. Also her gas mileage on that trip was 13 mpg driving at 45 mph in 4x4. The next fill up was 16, now back to the 17-18 range.