The computer tank average reading has been inconsistently not accurate to me. I have tested the computer reading against ACTUAL calculation (full trip miles / full tank gallons) for multiple times through couple months. The "tank average" is almost always 1.6-2.2 HIGHER than the actual one.

To be more specific, I do 50% city / 50% freeway but both with bad traffic in Los Angeles. The tank average almost always read around 24-26 MPG but I usually get 22-24 in actual calculation. The most frequent tank average i got from computer is 25.1 MPG while actual is 23.5.

I find it more accurate when you guys report by the TRIP MILES at the time the gas station icon pop up (assuming you reset the trip miles every time you refuel). Mine is around 310-325 miles or so. What is yours?

Note: This is on IS250 2012 with navigation and F Sport package.

 

This is a moot topic. And you're going to get many different answers..

(1) do you fill your tank until the pump stops, or do you "top off"? Every pump is different. The fuel gauge can't read "over filled" levels.
(2) the tank average is just an approximation
(3) what "gas station" icon? do you mean the low fuel light?

 

I don't ever let my tank get that low

 

My low fuel light comes on around 360 miles. Im usually around 20 - 24 mpg. I fill up at Chevron if that matters.

 

There are numerous threads on gas mileage..

 

Below 280 miles; then again, I only fill 1/2-3/4 tank at a time

 

The mpg average for my car is 25.8, and the fuel range is about 365-70. I always reset the Trip A to 0 when I fill up. I find that the fuel range and my actual miles shown on Trip A are usually very close and only a few miles off. I don't care for the Tank Avg screen as this jumps around. Best thing to do for calcuating actual fuel economy is using the trip computer and basic math, works for me

 

@TimboIS
(1) I fill it until it is full just like many others do.
(2) Exactly! When i do the ACTUAL calculation of MPG which is (total miles driven before last fueling) / (total gallons used) . The "tank average" is almost always 1.6-2.2 higher than the actual one.
(3) Yep, low fuel light is my reference.

 

May I ask how you got about 20-24 MPG when the low fuel light comes on around 360? If you get 360++ miles per tank, you should get more than 24 MPG??

 

Exactly! I'm trying to compare the computer "tank average" VS. MANUAL CALCULATION and in my case, the computer "tank average" is higher than the real one by 1.6 to 2.2 MPG. In another words, Lexus reading seems to be more optimistic which make me think i'm getting 25+ MPG while i'm actually getting lower than that.

 

Actually the tank average display is consistently inaccurate. Mine always reads about 2mpg higher than what I calculate from tripmeter miles divided by gallons to fill, but because it is consistent, it's easy to just subtract 2mpg from the display to estimate the actual mpg.

You can't count on it to be perfectly accurate. I don't think it can even accurately measure how much gas is in the tank. Maybe if your car is on a slope, or leans to one side because the only weight is on the driver's side, then that could throw off the tank level.

The fuel light comes on when you have around 4 gallons left in the tank. I can usually do around 375 miles to this point. Of course, the number of miles varies depending on your driving style.

The range reads 0 when you have around 2.5 gallons left in the tank. I have driven past this point many times to 450-487 miles, yet the most I have ever needed to fill the tank was 15.869 gallons, meaning that there was still 1.3 gallons left in the tank.

Here is another thread for you:
How accurate is your trip computer on MPG?

 

My tank average is pretty good with my calculations. My display usually shows 17 MPG after refilling full until the range show 0. I also reset the trip display after every fill-up and rarely do I see anything over 300 miles. Most of the time it's about 260-280. Refilling from "empty" is about 15-16 gallons for me. So that's about 17.5 MPG in reality. This is with an 07 IS350 in Chicago traffic. You get stop signs every block and lights every other, plus Chicago drivers are aggressive drivers in general, when the light turns green they take off and people tailgate you if you're going only 30 MPH.

 

I check the tank mpg once when I saw in another thread that someone said it isn't accurate. From what I can tell, my car estimate is fairly accurate with about ~20 mpg in mostly city.

 

I average ~24.7 MPG mostly urban diving, IS 350. Displays range of 365 miles usually after a fill.

 

Most of my tank averages are about 1-2 MPG off compared to my manual calculation. My lifetime average is 27.7 and my average tank is normally between 28-31 (60% highway and 40% city), and my manual calculation is normally between 26-29. I <3 driving slow