my wife's highlander only gets between 14-15 mpg in winter... not sure if I am understanding your question correctly.
in cold weather , short trip reading 12.6mpg sounds right to me..

I calculate the mpg every time I refuel and number on computer is almost accurate.. I see less than 5% variability.

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12.6 is quite low for the 8speed HL , I take it easy when the engine is cold in cold temperatures and the vehicle is garaged so temp at start up is not as low as a vehicle parked outside. I notice though on start up the read out starts by default at 99.9L /100km or 2.35 mpg and works it's way down over the next minute or so down to the expected figure, so that contributes to a some inaccuracy on short trips I would think. Not an energetic driver,try to drive as smoothly as possible and around the speed limits.

 

I don’t see how one can accurately determine fuel economy for any period or trip that lasts less than one tankful. I’m pretty sure there is no fuel-flow sensor measuring how much is being consumed minute to minute. The amount of fuel that is being used is determined by measuring changes in the fuel gauge reading. The computer is giving you an estimate. The only way you really know is to fill up the tank and divide what went in by the miles driven since the last time you filled it up.

 

The dash display is not accurate.
It calculates based on injector pulse width. Fuel temperature, octane, and ethanol levels have an impact.
My Corolla typically reads about 2mpg high over an entire tank, but sometimes it's dead-on. :dunno:

 

Thanks for actually reading the post, yes I agree there is some error as on a second similar trip it was reading that same high 20L/100 km plus but this time the vehicle was turned off for a while and on the return journey, similar return to the earlier trip, it got down to 12.9L/ 100 km, so it is probably accurate over a tank full or more , it does have quite a bit of error in colder temperatures. I notice it updates every 10sec and this is more than just math as the reading that I experienced did not improve even when cruising at 40 mpg with few stops.

 

It's just a guess based upon the the distance traveled divided by the estimated fuel used in the denominator. The larger the numerator, the less effect a short trip will have on the longer-distance estimate.

If you had 10,000 miles reading 17 mpg, and you burned an entire tankful of gas by sitting with your foot on the brake and CREEPING forward only 1 mile during that entire tankful of about 19 gallons, therefore getting getting about 0.0526 of a mile per gallon, your average MPG for the 10001 miles would be about 16.6.

Not real sure what your point is...it's just math.

 

Another factor is where are a lot of the electronic components in the Indiana produced HL sourced? Through US suppliers? And if so do those suppliers source components from elsewhere? I have noticed a few somewhat unusual things with the systems in the vehicle, one being the wash function with the wipers where on occasion I get wash without any wipe. Others have had door lock acuators faill almost unheard of on a Japanese built vehicle, others have had their HVAC not stay in auto mode, these sorts of issues are not Japan spec issues. Japanese electronics are almost bullet proof.

 

OK did a short city trip including going thru a drive thru, -15 again, startng from cold no warm up, gentle driving and 17.2L/100km or 13.7mpg Short time later, engine still warm did 10 mile trip on typical rural highway, keeping to 50mph speed limit and total of 4 intersections requiring a stop, this time 9.9L/100km or 23.7 mpg, that is a huge difference in fuel consumption, the HL is very much a highway hyper miler.

 

No, it's just heavy. Physics, that's all. It certainly isn't a hyper-miler.

My 2018 Camry, rated at 39 mpg hwy can get well into the 50+ mpg range cruising on the highway traveling at the speed limit with 3 adults in it. For a 200+ HP normally aspirated 4 cylinder engine, that amazes me.

 

When I said mine was accurate, I live in the north Georgia mountains, so mine is all country miles but they are also up one mountain then down the next one. The lowest mpg on the dash has been 20.1 and most of the time it’s 20.6.

 

Per tankful, I have found the readout to be accurate enough not to bother with calculating it manually. I still don't understand the point of looking at the estimate for a 10 mile trip since there's no way to know what the actual fuel consumption was. One estimate is probably more accurate than the other but you really don't know which one. They might both be equally inaccurate.

You could do the same trip 30 times and get an accurate manual or trip-meter reading but isn't that what a tank-ful reading is?

 

Per tankful, I have found the readout to be accurate enough not to bother with calculating it manually. I still don't understand the point of looking at the estimate for a 10 mile trip since there's no way to know what the actual fuel consumption was. One estimate is probably more accurate than the other but you really don't know which one. They might both be equally inaccurate.

The shorter trip gives a snapshot as to what type of driving and conditions impacts the overall figure. Why am I getting overall 17mpg?By breaking it down into these smaller snapshots from start up I can see the issue if the computer is somewhat accurate and in my case it seems to be the amount of stops I have to do coupled with cold temperatures, the really cold temperatures cause quite an impact to the mpg when starting from cold and there is enough of these conditions and driving to pull my mpg down. Normally I just have the computer set to from last reset, but this was a little experiment and if some find it of interest and want to try the same thing no harm done.