Ive been filling up my tank with 91 oct and have been averaging 353 miles on full tank

Yesterday i filled up my tank again with 91 oct and my range is only 313

Does anyone know why i could have lost 40 miles?

 

Not enough information to determine... How many gallons did you put in the first time vs how many gallons the second time? The better measurement is to divide the miles by the gallons put in to see what your MPG is. Then you could have an apples to apples comparison. I don't think I've ever stopped and put the same amount of gas in both times.

 

He said "I've been filling up my tank" so it shouldn't matter how many gallons he put in, as long as he actually filled up. He is talking about the range the computer gives once you refuel, so fundamentally if you fill up each time you refuel you should have about the same number of gallons available for the computer to do the math.

On the other hand, if he wants to know if each tank at empty gives him the same MPG, then your theory is spot on. He just wants to know about the car computer's predictions at full.

 

I think we should keep in mind that the "range" given is only an estimate based on parameters that Lexus/Toyota programmed and mostly based on history/past data registered. I think a lot has to do with the car's previous 'experience'/statistics leading up to the most recent empty tank, and then the computer does its best to predict a "safe" estimate of how many miles you can anticipate from the fill-up you just did. The statistics up to the last empty tank probably include such as driving habits experienced, adjustments the car made based on the quality of gas used, altitude, air density, oxygen amount per given sample of air, engine load from people/luggage in the car as well as from such as A/C, and other electric using components/accessories, etc.

Now, for your situation; my theory would say that it's possible that on your previous 353-miles-range tank you maybe used the A/C on a rather warm day or two since spring is in the air, maybe rolled down the windows on cooler days causing some drag, and possibly did some 'spirited' driving down the road coz the weather's just too nice - coupled with some hard-braking, or probably used a different blend of gas (different gas station or brand or blend since winter is over) etc. you get the point, and then the computer made the adjustments on your recent fill-up that it thinks you'll get just about 313 miles. This would probably hold true if you drive EXACTLY (or close) the same way as your previous tank(s) and under the EXACT (or close to) same conditions.

I always reset trip A at each fill-up and the given estimated range at the end is sometimes close and sometimes way off. I once had an estimated range of over 495 miles because my previous tank was spent mostly on the highway, I only got 445, I once had an estimated range of 385, but got 430 before light came on. Of course it adjusts as you drive, sometimes I drive for 4 miles without any reduction on the estimated range. By the time it gets to 40 miles range it's typically safe to assume you should fill-up otherwise you'll be stranded if you plan to drive more than 40 miles.

My 2 cents.

My car seems to use 20 gallons before I have to fill-up, and in your situation that translates to a 2MPG lower prediction on this tank you just filled up.
Moral of the story, the lower predicted/estimated range doesn't mean you just put bad gas in your tank - the car doesn't know (yet). And it doesn't necessarily mean your O2 sensors are failing (even though over time this could be). Chances are, if you drive conservatively you'll get your typical 353 miles or better even though the computer says you'll get 313.

 

Thank You Wise.

Much appreciated.

 

OK.. Wise2k hit it on the head. Very good description. I totally missed the 'range' part.

 

On a recent road trip I got apprx 560 miles out of a (very) full tank of gas. All interstate. My indicated range display after fill up is about 425...not sure what the exact number is on my car but it is pretty conservative calculation.