I keep every gas fill up record and mine dropped when I moved to Chicago from a suburb....

went from 20mpg avg down to 16mpg.
I get 30 highway though (give or take a few).

 

I calculate my mpg between fill ups and always average 21. i drive fast, even WOT at times. 100% rural driving (guys, whats the word here...obv not city because its ol Pennsylvania country roads) but yeah you get it.

 

went from 14-16 city/highway and 18-20 highway to 18-19 city/highway and dunno yet will find out on road trip this weekend my highway since cleaning my throttle body

 

Went on a road trip about two weeks ago. Car has synthetic oil, and fresh Amsoil in the tranny. I also put a double dose of injector cleaner in the tank. Tires are stock 17inch pumped to 34psi.

I got 710Km on 61 litres of gas. That works out to 29mpg. I was averaging about 75mph with a few hits around 80-85 and some city driving also. This is the best mileage I ever got. Usually I get around 25mpg

 

GS3 owner here, but they get fairly the same gas mileage. I usually get 19-21 mpg.

 

Currently getting 29mpg on a tank of 93 Octane 100% gasoline (Non-Ethanol), 70% Highway, 30% City.

 

Update, Tank Ended up at 28mpg. (436mi, 15.6gal spent)

This was with some fairly heavy Zyoxx 20" Wheels with Nitto Neo-Gen 225/35/R20 (Slightly stretched to fit the wheel)

Trying the same fuel with stock 16" Platinum Series wheels, might hit 500 miles with it which would be just over 32mpg average!

 

well update...i got 25.6 mpg driving at 2,200 RPM or 70 mph on ALL highway probably coulda got better if the last 20 mins i hadnt have driven 100+

 

My GS400 has averaged just slightly over 20mpg over the course of the last 4 years and 28,000 miles (140k total mileage). I get in the mid to high 20's on highway trips but I do more city driving than highway, the car has no mods and I check the tire pressure frequently.

 

Do you know if the fuel was E10? or pure gasoline?

Stock wheels?

I've gotten 29mpg on a tank of straight 75mph driving before (elevations of 2,000-4,000ft, steady rise over 400 miles)

 

These are great numbers. I went on another road trip this past weekend and so far have 250 miles on a little over a half of a tank. The last half seems to go fast but it looks promising.

So far I am a believer of Amsoil. I have never seen 23mpg+ in the 6 years I have owned the car!

 

My average is 21MPG all around (70/30 city/highway mix)
Personal best over 445 mile trip prior to this week was 28.6MPG (75-80MPH with AC on) I just repeated the same trip but filled up after the first 301 Miles and got 26.48MPG (Cold weather 28-32 degrees) on the second 144 miles I average 37.27MPG. I believe the cruse control set to 65MPH for most of the 144 mile stretch made a big difference (65MPH was not by choice it was a Seasonal Speed Limit Strictly enforced through a section of I-80 in Wyoming) this bring my personal best to 29.21MPG over 445 miles.
Not bad for a 11 year old card with 123K...

 

18 MPG a gallon if driving only in city, 21 MPG on highway...98 GS 300

 

Checked recently. 14 mpg city. I've changed the O2 sensors since then. The CEL and the fuel smell in the exhaust is gone. Anxious to find out if the mpg has improved.

 

was pure gasoline.
stock wheels

that seems low for a gs300