We have a 450h that does better but from what I recall when we would get 350s as service loaners that sounds in line with our experience.

 

Sound like you may want to change out the old spark plugs and put in a new engine air filter to allow your engine to burn fuel more efficiently. However, I would not expect much better gas mileage from a mid-sized crossover with a large 3.5 V6 and a six speed automatic transmission. The 2010-2015 RX350 is the wrong vehicle to purchase if fuel economy is your main concern.

 

My 2015 RX350 AWD average about 18-19mpg with my heavy foot(80+mph on the highway) on my 30 mile commute to work that includes city and highway driving. I did not know if the spark-plugs are new or old, but I did install a new Spectre long life air filter. I am good with the gas mileage that my RX350 is making. Hopefully, the gas mileage will get a bit better when I change the spark plugs, transfer-case oil, and rear differential oil.

Note: My RX350 AWD does have larger than stock 20in wheels with wider than stock 255/45-20 tires. I only use BP 93 octane.

My current gas mileage after my commute from work.

 

This is about what I get for city driving with either a stop sign or traffic light every or every other block. I wonder how different it'd be if I changed out my spark plugs.

 

As most of my driving takes place in the city (Montreal), I would expect my fuel economy to hover around 16-18 MPG (13-14 L/100 KM). However, no matter how I adjust my driving style, I cannot seem to get better than 12-13 MPG (19-20 L/100 KM). Whether warm or cold outside, uphill or downhill, after refueling my gas mileage reading eventually returns to this number. While I see others on here claiming a minimum of 275 Miles (450 KM) on a full tank, I get just under 400 KM every time. Since I purchased the vehicle in 2017, I have used 87 octane during every fill-up. Could there be something wrong with the car? Do I need to be using 91+, or is this the reality? Fuel system cleaner? I understand I will never get EPA rating, but this far off is becoming annoying to fill up so often. Any feedback or advice is highly appreciated. Thank you!

 

So what do you have for mileage? Sorry if when I skimmed I missed that, but you could be due for new spark plugs at this age and a new PCV along with maybe coil packs.
That sounds like the most likely culprit, to me. I have 230,000km and just cooked an alternator, but my worst mileage in the dead of winter with lots of warning up the car and terrible fuel full of butane to disguise it as #WinterGas is like 13L/100km.
Something is mechanically wrong unless you drive like a total Koont.

 

For your city driving, how far are your trips? If most of your trips are under 4km/2mi, that type of gas mileage would be understandable.

 

Thats Crazy! big diff with FWD vs AWD i guess. Also my 20" wheels must be real heavy. I have 280k miles.

 

I lost the plastic that goes underneath, not the first foot but the second. Could that also affect my MPG much or negligible

 

I took both of mine off to find a rattle months ago and never bothered to replace. When I add my two dogs in the back (140 lbs) I lose 2mpg on the highway. City about the same. Otherwise I have not noticed a difference in fuel economy.

My overall after having a 2010 rx350 awd w/ 173k miles for the last ten years is 18-20 mpg in town. Depending on lights and lead foot. 23-24.5 mpg on the hwy. Depending on wind direction. I get the best mpg on 45-55 mph state roads. There I get 25.5-26mpg. Still on original spark plugs. I put Kroger gas in it always, unless on the road. Flip flopping between 87, 89, and 93 octane. Depending on how many points I got versus the price of fuel. I seem to get the best economy out of 87 octane. Even though car says to put premium only in it.

 

We traded in our 2013 RX350 AWD w/18" wheels a couple of weeks ago, on an RX450h+. It had a bit over 160k miles on it and still ran perfectly.

The average was 23 mpg with mostly highway. Best ever was 27, though 24-25 was more common on long road trips. It would dip to 17-18 for city driving.

 

Just got 24.5 on a 400 mile highway trip, based on actual amounts from the pump. I topped off when leaving, then filled up at about 200 miles, after 60-70mph roads most of the way, and averaged 25.9. The rest of the trip was at 75mph @23.3. That surprised me a bit, as the Since Refueling average always takes a dive when getting on the faster roads. I also think the on-board measurement runs about 1 mpg low.

Still 16-17 in pure city driving.

2010 RX350 AWD, 241,000 miles

 

About 20 in the city and 26 on the highway. 2012 RX350 FWD.