tbartman67

Since I bought my 2024 NX350h last summer I have been diligently recording my numbers every time I fill up (which isn't that often) - miles since last fill, gallons added, "mi left", etc. - mainly with the goal of figuring out how many miles I really have left when it says "0 mi left." FYI - on that, I have found it has at least 2 gal left in the tank when it says 0 miles left (it's supposedly a 14.53 gal tank and I've never gotten more than 13 gal into in, even with one extra pump after the pump stopped, and driving it 19 miles beyond "0 mi left"). When I do all the algebra, with my current mpg since the last fill-up, I calculate I have about +/- 70 mi left when it says 0 mi left (range from 42 mi in the winter when the engine runs more, to 98 mi in the summer when it runs less).

But the point I want to raise here is that the car shows my mpg since last fill-up (I always update the mpg on each fill) and it is always high by about 1.5-2.0 mpg! Every single time. For example, I just drove 379 since my last fill-up, put in 12.45 gal, so that's 30.44 mpg. But when I stopped for gas, the dash said 32.0 mpg since last fill-up. You could say that's because I did the extra little pump, which increases the denominator in my calculation, but I do it the same way every time, so it should even out in the end. In other words, if I added "too much" gas one time, I would calculate too low a mpg, but then the next time when I didn't over-top as much I would calculate too high a mpg. But this never happens.

TBH - this time the pump stopped for the first time at exactly 12.00 gal, so even there I'd get 31.6, not 32.0.

Any ideas (or corroboration) that the car is telling you you're getting a better mpg than you really are?

dklanecky1

In my opinion, the incredible variation possible in vehicle MPG results due to vehicle condition (tire pressure, air filter condition, HVAC settings, gasoline quality/grade, weight of load in vehicle, for example) and driver style (acceleration rate(s), braking rate(s), cornering rate(s), actual driving speed(s), for example) and driving conditions (season of the year, weather factors, uphill, downhill, road condition, gravel or paved, etc.) makes ANY precise predictive calculation of MPG's an impossible task.

Almost the same as how many miles can I drive on a tank of gas, and /or how many gallons does my tank REALLY hold?

Just relax and enjoy the ride, fill it up when it hits 1/4 of a tank and forget about trying to make function it like a cell phone which tells you in (allegedly) 1% increments how many minutes you have left before your out of battery.

tbartman67

I'm not asking it to be precise when telling me how many miles I have left. I know they always err on the side of caution so some idiot doesn't sue them when they run out of gas on the highway when it said "1 mi left." I now know I can safely go at least 40+ mi when it says 0, when I'm not near a gas station.

However, you say it shouldn't be able to do a "precise predictive calculation of MPGs." I'm not asking it to predict my future MPG. It SHOULD be able to tell me my EXACT MPG since the last fillup (or last time I reset the counter). It knows how much gas it's used, and how many miles I've gone. If I fill the tank, reset the MPG counter, drive exactly 300 mi, then refill the tank to the same exact spot with 10 gal of gas, the dashboard shouldn't be telling me I've been getting 32 mpg on that tank when I rolled into the station!!! But that is exactly what it's doing, every time I fill up. That's wrong.

kent2174

2024 NX350h Premium, purchased in August.
here are my last 5 fuel fills:
car stated 35.15; actual 35.0
car stated 34.57; actual 34.6
car stated 33.00; actual 33.00
car stated 37.4; actual 35.3
car stated 39.1; actual 40.07

tbartman67

kent2174 - thanks for the reply. You're dead on almost every time, except one where you were also off by about 2 mpg like I usually am. Again, this would happen if you put 'extra' gas in the tank, filling it higher than the previous time. I do see that the next value you were higher than the car said, so that makes up the difference. I wonder if you added past the first click on the second to last line but stopped at the first click the final time. I just NEVER get more than what the car says.

I did find other threads here that say the car overestimating by 1-2 mpg is fairly common across models and conditions.

midcow3

Different shut-off at different pumps. Can't expect the accuracy you are trying to achieve when the input data is divergent and not consistent to the degree of accuracy you are trying to calculate. And yes, when DTE = 0 and no bars Lexus provides a 2.8 to 3 gallon reserve.

By the way this issue has already been discussed a whole lot in previous threads.

YMMV,
MidCow3

anoop

Being consistently 1.5 to 2 mpg off means something is wrong.

Are you filling at the same pump each time. (Still, even if not, being consistently off seems like there's a problem.)

It's probably worth documenting it and asking the service dept to check it when you take it in next.

If it were my car, I wouldn't even know. I stopped tracking that when I cars started doing the calculation for me.

As regard to tank size, car makers are known to have the concept of "reserve". Some autos include that in the reported tank size (which is what Lexus seems to be doing), and some don't (in which case you get a surprise post saying "I filled more gas than the tank size"). It sounds like the NX has an ~2 gallon reserve? [Anyone know for sure?]
https://vehicleuniversity.com/do-car...reserve-tanks/

As an aside, 32 mpg sounds low for a car that is supposed to get almost 40 mpg. That is something I would be upset about.

tbartman67

With all due respect, I get that the shut off may be different at different pumps. But then sometimes it should be low and other times it should be high! It's always the same direction.

Let's say, for example, in reality I always get 30 mpg, and that's what the car tells me every time.

Tank 1 - I fill it to the tippy tippy top. I drive 300 miles. I actually use 10 gal. When I go to fill next time the pump shuts off a little early and I only put in 9.5 gal. I think I got 300/9.5 = 31.6 mpg
Tank 2 - I drive another 300 miles. I actually use 10 gal. But this time when I truly fill it to the tippy tippy top again (where I started the first time), I get in 10.5 gal (since I stopped 0.5 gal short last time). So this tank I think I got 300/10.5 = 28.6

One is higher than what the car said. One is lower. If I added them (600 mi on 20 gal) I would match the car's reading.

What I am saying is after 10 tanks, my math comes out ~2 mpg less than the car says every single time. That's not insignificant - it's about 6% difference. The only way this can happen is if gas is leaking out of the tank, my tank has magically expanded by many gallons over the last 10 months, or the car is not telling me the true MPG.

Back to my last post - this website did suggest a few other threads to look at, and there are many others who have documented this. I'll just know not to brag about the MPG that I'm seeing on the car's display when I know it's not accurate.

tbartman67

Anoop - over the life of the car I've gotten 30.9 mpg (109.85 gal for 3393 miles). The difference between summer and winter were dramatic. I drive only a few miles to work in the city, and I bet most of the fuel savings is when the engine shuts off at stoplights. In the winter it never does this because the engine never gets warm by the time I get to work. TBH, my last car was a 2010 RX350 and I think I was getting about 12 mpg because the car was sitting still with the engine running >50% of the time. I have had a few fill-ups where by my calculations I got ~35 mpg.

I did use the Eco mode for a while, but switched to Normal because it was seriously boring to go from 0-35 mph and have to use a calendar to measure how long it took - LOL

midcow3

I am not sure what kind of answer you are looking for ?

midcow3

That is a completely untrue exaggeration. There is some difference between modes , but if you press the gas pedal all the way down ( floor it was the old term) it accelerates at the same rate as other modes. You have maximum ICE and maximum electric motor.

AM123

So here is my humble knowledge on this subject. Firstly, every car computer fuel economy information is different and we need to know how to interpret that based on the manufacturer’s owners manual. Speaking in particular of NX350h, the information on the display says “ Total Average fuel economy”. According to owners manual, this means “Average fuel economy since starting”. See below for the screenshot straight from owners manual. It also clearly explains the distance to empty may vary based on number of conditions.


That being said, I always use the simplest means of calculating the MPG by odometer method. See this link for steps to calculate MPG : https://www.fueleconomy.gov/mpg/MPG.do?action=calcMPG

I always go by the odometer method and life is good. Here is latest MPG,

Odometer difference was 496 miles and I filled 10.33 gallons which equals 48 MPG. Cheers 🍻 and have a nice weekend folks.

midcow3

Actually, your statement "Firstly, every car computer fuel economy information is different" is not really correct. There are only a couple of variations: (1) average mileage sense reset (this is what Lexus NX uses), (2) average mileage since start of this trip, (3 )average mpg since fill-up, (4) instantaneous mpg

Lexus is a little behind other car manufacturers and only provides (1) above average mileage since reset. You can reset but then you lose your mpg history.

DTE is basically correct as state. Yes it is based on previous driving mpg ( we don't know Lexus's formula). However, if you been driving and getting high mpg your DTE will be greater than if you are hot-rodding your NX, to the extent you can aggressively drive an NX.

Where Lexus is way too conservative is that DTE and gas gauge bars are based on a tank size some 2.8-3.0 gallons less than the actual gas tank size of 14.5 gallons. Lexus calls this a reserve, because most Lexus drivers they think are too stupid to look at the gas gauge or DTE and would run it down to zero and run out of gas. So in effect you have an effective gas tank size of not 14.5 gallons, but only 11.5 to 11.7 gallons. On an NX 350h rated at 38 mpg, this is a range of 448.5 to 456.3 miles.

The OP, @tbartman67 has asked a lot of different questions and and made a lot of different observations, that the mpg show is 1.5 to 2.0 mpg higher than actual based on his calculations dividing miles driven, by gallons added to fill-up. This could be considered a Lexus calibration error or Lexus might shun it off as accurate within normal deviation and that operating as expected. Good Luck trying to get Lexus to fix this. Lexus denied me a request to accurately adjust the gas gauge and DTE to be correct of 0 miles remaining when DTE is 0 and there are no bars. They told me that a reserve of 2.6 to 3.0 gallons is normal and that they could not and would not make any change. Then the brilliant Service Manager had the gall to say, 'Why would you want to run down to 0 anyway?" The Service Manager completely missed the point of accurate reporting.

Mileage data are subject to some variations, such as tire tread, tire air pressure, fill-up cutoff consistency, temperature(volume) of gasoline, etc. Therefore calculating the mpg to tenths might now be accurate because the input data are not consistent.

There are a lot of threads already discussing mpg and gas gauge and DTE accuracy. These might help improve your level of understanding of the issues,


YMMV,
MidCow3

AM123

Interesting! I guess, no comments since you mentioned "Lexus denied me a request to accurately adjust the gas gauge and DTE to be correct of 0 miles remaining when DTE is 0 and there are no bars. They told me that a reserve of 2.6 to 3.0 gallons is normal and that they could not and would not make any change ". I believe this thread seeking. The computer gauge is not clear to make the user to understand and hence why, I use simplest means of methods to calculate MPG.

I wish Lexus used this method "(3 )average mpg since fill-up,"

"There are a lot of threads already discussing mpg and gas gauge and DTE accuracy. These might help improve your level of understanding of the issues," this is true but searching for the exact thread you are looking for is not easy or straightforward (at least for the newbies). Hence why, I try to navigate people to similar threads or at least share the information.

Cheers 🍻

SeismicGuy

I noticed thata "shut-off" was mentioned a couple of times in previous posts. I assume you are talking about when the pump nozzle shuts off by itself when fueling and this is one of my small peeves with this vehicle. With most vehicles I can always add a bit more after the pump auto shut-off. With the NX it seems to shut-off long before it needs to and I can always add quite a bit more fuel. Maybe has something to do with the shape of the tank or the filler tube?