14.5 City and 17 Hwy using 87 octane on my 04 with 84k miles. Not bad considering my wifes GMC Envoy with the 4.2 inline 6 cylinder gets almost identical gas miles.

 

the GX is tuned to use 91 octane. By using reduced octane the ecu is having to retard timing, which in turn gives you less power and fuel mileage.

Will this hurt the engine? probably not
Just dont expect to get the best mileage out of it using the lower octane.

 

I have ponied up the extra money on 93 octane and didnt notice extra miles. I also cant notice the performance difference either so to me I might as well save the $5 per fill up.

 

yeah, it's difficult to accurately guage if 1 tank of premium benefits over regular because the paths taken are not equal, among a host of other things.

If you drive the premium tank mostly up hill and the regular tank down hill, you'd be convinced never to buy premium again, and vice versa the other way.

In any case, buying premium over regular only costs me about 6 dollars more a tank, which doesnt break my bank anyway.

If you require water, but always got coke instead, do you suppose you'd feel as good as you could?

 

it says premium fuel only on the fuel door. git er done.

 

I think in the old 03/04 GXs it doesn't matter much on the octane rating. They're relatively low compression engines and wont make much difference. The VVT-i 05+ I think would have a greater impact on fuel economy and power delivery. FWIW, mine doesn't say "Premium only" anywhere. It Recommends 91 in the manual but its not required.

I also get ~ 15/17 for fuel economy. Its on average with others.

My Computer says I get 18.3 MPG average though, LOL.

 

I would check what the previous posters suggest.

Incidentally, I am getting 1+ mpg better using ethanol free gas. I use www.pure-gas.org to find convenient stations. I had been getting <16mpg and am now getting 17+ mpg (calculated).

2004 w/ 91,000miles

 

To the original poster, have you done your spark plugs recently? Or ever?

 

last tank got 14mpg.

premium, chevron/shell. again, i am running 285/18 AT tires.