just wondering when i filled up with gas and then calculated from the miles on odometer / gallon used i get a different reading than what is on the trip computer screen. So lets say i got 18.5mpg from traditional way but on the screen display it says 20.3mpg. why

 

are you wheels and tires stock size?

 

The on screen dislplay is a calculation based on airflow readings, injector duty cycle, engine rpm and wheel speed sensors... its accuracy will vary, but it is never 100% sometimes it will be dead on, other times it may vary a small amount, as yours did. One other factor is making sure the exact same amount of gas was added... Some pumps stop earlier than others...

 

hmm i see. ok this was my 1st time so i will keep comparing like this for few more fillings and see. funny thing is that i also did same thing on the Volvo S80 and i also got different readings as well.
I still think 20.3mpg on the Rx300 is high compare to what other RX300 owners get. Plus i havent done any IACV cleaning or change PCV. It has approx 100,000miles on it. Just had air filter and new tires like few months back and put in regular fuel from Shell.

 

thats great to know how its calculated. this 100 degree weather in the midwest has been taking its toll on my MPGs.. I've driven about 2500miles this week..

North Carolina back to Illinois for the 4th vacation i took to see the family. Drove from central Illinois to northern Illinois. And drove to Ohio the next day. 24-25mpg. when it was about 80 driving around Ive gotten 28mpg. Plus I've been getting better consistant mpg running midgrade 89 octane vs 93/92. on my 99 Ls400

 

computer screen is never accurate.

 

One thing about fuel mileage... you should generally get the best mileage using the recommended fuel for your car. What they don't really advertise though is that they may use higher percentages of ethanol to get that higher octane rating... e85 requires 25 to 50% more fuel to make the same power, so it should be obvious that using a higher percentage of it in normal fuels will have an effect as well...

 

To my knowledge most newer cars equipped with knock sensors the electronic control modules can advance or retard timing depending on knock count. I believe the LS400 was tuned a bit rich. reason why I can run mid grade and get no knock and get better MPG than premium.

Modified DSM and Honda guys have been using the knock sensor technology to tune cars.

I have a friend with a dyno. I might just strap the ls400 to it and see how it does on different fuels.

 

My RX's average mpg read-out is usually very accurate, within' .3 of the actual mpg.

I have yet to determine how accurate the read-out in my ES is considering I just bought it and haven't even burnt through the first tank of gas yet.

 

Under part throttle conditions, every car is tuned about the same - to follow the O2 sensors... There is a base tune, obviously, but as every ECU learns, it adjust fuel trims to the pre-programmed tables to run as stoich as possible under part throttle conditions. Of course, it throws that all out the window when you floor it, though it still does use its long term trims to adjust the open loop table as well...

In general, as we all should know, lesser octane fuels have more power per unit than higher octane fuels, but at the expense of potential knock.

My SC400 (exact same engine as your LS400) got better fuel mileage with higher octane fuels, when I lived in the the southwest... Here in the mid west, I know for a fact that many people get lesser fuel economy with some of the higher octane fuels, depending on where they purchase their fuel...

For example, a local gas station chain has 87 and 89 octane fuels that cost the exact same. The difference is that their 89 has a higher percentage of ethanol in it. Because ethanol has an even lower power per unit than "regular gasoline" running their 89 vs their 87 will lessen your fuel mileage.

I run E85 in my IS300 (tuned for it obviously) and it requires 30-45% more fuel, depending on load and RPM. As a result, my fuel mileage sucks. I don't know how bad it is, but it is less than it was prior to E85...

 

what i didnt know that ..i did notice it that on my RX i switched to 93 (highest) from Shell..and it actually lowered mpg compared to medium octane..weird.So what you are saying is fill it up with lowest octane to get high mpg? or octane with higher ethanol gives bad mpg that with no ethanol?
Also on my Volvo too i had same situation..higher octane didnt improve mpg .i will check the ethanol % the next time i will fill up..

 

You want to run what is recommended for the car, if at all possible. Higher octane will not help fuel mileage on a car designed to run on lower octane. Higher octane may or may not change fuel mileage on a car designed to run higher octane. I have noticed in a few cars that using the proper octane (which is generally 91 in most Lexus') helps maintain good fuel mileage...

To take octane out of the equation, if you have 91 octane from two different gas stations, you will get better fuel mileage from the gas station that uses less ethanol to obtain their 91 octane rating... The downside is that the gas station that uses more ethanol probably will have better prices. Here in town, our average regular fuel price (87) is about $3.25, with 91 going for $3.45-$3.50, depending... I pay $2.99 per gallon for E85... I can also get E40 for $3.15 a gallon...

 

I've been mixing mid grade and premium back and forth..

my o2 sensor is telling me my cat is bad. running on straight premium the check engine light is on. running straight midgrade the check engine light is on. when i mix it the check engine light goes away after xxx amount of miles.

from what I've heard.. gas is shipped at a higher octane and when it gets into the ground it dilutes.
so really the octane levels could be higher than stated just by a little or less than stated ..

my car has been in Kanasas City all its life. and the previous own ran premium.

I've gotten better gas mileage running on midgrade coming back from OH on Saturday. I drove there on premium.

 

I wouldn't say it dilutes, but fuel is constantly losing its octane rating, albeit very slowly... very weird about the mixing fuels and the cataalyitic converter, I would say that is more of a coincidence than anything. Octane rating has little to do with how well the cat works...

 

Also, there are so many other variables. Somewhat recently, I went to pick up something and used my dads diesel truck... On the way there, I averaged 16.5 MPG, on the way back, 18.5. Exact same roads, speed, etc. Load was essen tially the same, I actually picked up weight, but not much... Big diesel truck, about 30 gallons to get there, 27 back, essentially no changes, including fuel...