I have a 2014 GS350 with AWD 57k miles.
my brother has a 2015 GS350 AWD with 100k, my car almost never gets above 15 mpg overall unless I take a long road trip but even on the highway I usually only get 20-21 mpg, my brother routinely gets about 3 mpg better than I get and he drives far more aggressively than I do, I’ve gotten new oem plugs, air filter, and new tires. He has gotten the fuel pump recall done and I haven’t yet so idk if that would affect it, but I’m not really sure what else to look for, is this normal in your guys experience? I had a GS460 prior to this and could routinely get 30 mpg on the highway and 17-18 overall.

 

I would check your long term fuel trim values on a scan tool to see if the ecu is adding additional fuel to compensate for a vaccum leak, 02 sensor wear or something else that would cause the additional fuel consumption.

 

Something definitely wrong with your car somewhere. I get about 20-22 driving like an ******* on a regular basis and have gotten 26-27 on long highway trips.
Get your fuel pump recall done first and go through a tank or two of gas to see if your mpg changed. If it stayed the same then it would be time to start digging deeper.

Do you use any additives for the fuel system like injector cleaner?

 

Your car sounds pretty similar to my '18 RWD with 23,000 miles. I get similar MPG city/highway.

 

My 2016 gets about 29-31 highway, maybe 16 streets, low 22-23's mixed which is like normal for this engine and car but I run on eco mode most of the time but I think there's something else wrong if you get that poor of a mileage.

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Something is very wrong. You should only be getting like 15-16 MPG if you're trapped in stop and go/urban traffic all the time. With like a 80% hwy 20% city mix I get about 21-23 mpg all the time in my AWD.

Hell, in the past year my lowest was like 18.7, and that was only because during the beginning of the corona BS there basically was no highway speed enforcement and you could literally drive as fast as you wanted. lol.... the GS350 highway efficiency drops somewhat at continuous travel above 80 mph.

 

Yeah, I have a heavy foot and still get 18 on the low end on city streets. Highway hits around 23 and long trips get up in the 30+ range cruising.

 

I tend to get 16-17 in my normal driving which is almost all city driving. I've been watching/documenting my mileage and trying different things to see what seems to change things. From what I've found so far, my general sense is that different driving conditions can make pretty significant impacts.

For instance, instead of taking a route to and from work that had a few more stop lights and a couple decent hills. By taking a route that had more of a gradual incline and fewer lights I easily gained some average mpg.

Recently, i had a client where I had a client where I ended up taking the interstate for about 30 miles. That take I was pushing 19/20 avg mpg most of the tank but then a few things toward the end dropped me down to about 18.7. I also recently started taking another slightly different way home that takes about 3 miles of street driving and instead takes the interstate. Between that and having the radar cruise control on another couple mile stretch I noticed a difference.

A few weeks ago I took the car to do some photos for CL. In spending about 15 minutes in two different parking garages (so 30 minutes total) the car dropped 0.6 avg mpg. (this was in the second half of the tank where the numbers don't tend to bounce around as much).

Mine is an AWD and from my experience in general with AWDs your mileage is definitely different than a FWD or RWD.

Mine is a '16 F Sport AWD.

 

Yes, I would have service the car. I have a 2018 350 F Sport and get around 20 mpg. Drive 100% in manual mode and spiritedly most of the time.

 

You're calculating mpg by miles driven by gallons you pump and not by what the infotainment mpg states correct? I think I routinely see the mpg on the infotainment be off by at least 1 mpg

 

I've been driving on the city and no freeway or highway and I'm only getting 17.1 mpg last I checked today. The car is fun the drive and I didn't buy it for the gas savings though.

 

I have a '13 GS350 AWD non F-Sport with ~61k on the clock - i'm usually getting around 14-15 MPG in city driving and around 23-24 MPG highway. Your highway numbers are a tad low, but if you're doing anything above 75 on the highway, I think the GS can be a little extra thirsty.

One possible trick could be just disconnecting the battery for 20min and then have the ECU re-learn your driving habits to see if that changes anything.

Check for any bad O2 sensors as mentioned by fellow member BMGS, maybe clean out the throttle body/MAF sensor, check to see if you have any exhaust leaks or a bad catalytic converter.

Any modifications to your car in terms of exhaust, intake, wheels/suspension?

 

Nope, for now my car is completely stock, I’m just going off of the instrument clusters reading on the mpg, I haven’t done any tests of my own, I suppose somehow the computer could think I get worse mpg than I do. I might ask the dealer to check it out but I’m sure they’ll say everything looks fine, the car drives perfectly smooth and has never had any check lights on the dash. My dealer can’t get me in til February for the fuel pump recall so it’ll be a while before I can test that theory, anyone gotten the recall done and noticed any difference in mpg?