Any thoughts on my suddenly approx. 30% lower MPG (on dash display) in the past week or two. Only known possible correlation is State emmissions test (using data port). A bad batch of gas? Just got back from the local Lexus dealer and they can't find anything wrong.

 

30% is huge drop.
I noticed a drop when heavy A/C was running in the last few days, but it was only 1mpg. At the rated 18/22, we're talking 4-6ish mpg loss.

But if the car is not putting out any trouble codes and tires and alignment seem ok, I'd check the thermostat was operating correctly. If the car thinks it's cold, car would use extra fuel.

 

When you press the INFO button to the AVG MPG selection and hold the INFO button at that selection more than a second, it should reset the average to 0.

Log your mileage, fill the tank again and manually calculate the MPG, while monitoring the display. (miles driven divided by gallons used)

With the INFO button, switch to the momentary fuel consumption from time to time as you're driving. It should vary with your throttle, like a vacuum guage..

Fuel quality could be a factor, as you suspect. Refill at a different station. Go with premium grade.

Reset the engine ECM by removing the fuse or the negitive battery cable. You'll loose the clock and radio presets.

We understand as an O2 sensor goes bad, fuel mileage is affected. No check engine (yet)?

 

If the air temperature rises, expecially when a heat wave hits the area, MPG can go down big time. I have noticed this in Mexico City, where just about any vehicle I drive will chew up gas when it gets hot outside.

 

Thanks for all the suggestions. I'll try them out. My 2002 RX300 has about 41,000 miles on it. The outside temperature has been hot. I just topped off the tank, due to rising gas prices, so it will be a while until I can try premium gasoline.

 

Just curious, what is premium going for where you live?

In Mexico, it is still running about $2.80 USD equivalent per gallon, but the quality is

 

Lexmex is right, the searing temperatures we've been experiencing recently can have a considerable effect on your fuel mileage. A second factor we are all experiencing now is the conversion to E-10 ethanol fuels that will increase fuel consumption a few points on its own.

If you will watch your trip computer "information" you will see a display of "relative" fuel economy - if not actual mileage. I'm not sure if this is a measure of actual fuel flow or engine vacuum, but either will indicate the relative fuel consumption of your engine under various conditions. A little observation of this gadget can provide a lot of information about your driving habits.

I recently found that my RX330 consumes MUCH less fuel on fhe freeway or in hilly highway driving with a constant throttle opening. Using the cruise control wastes a great deal of fuel as the ECM is prompted to accelerate to maintain a constant speed regardless of conditions. If you can hold a constant throttle position, letting the vehicle slow 5-6 mph going uphill and overshoot your target average speed on downhill sections, you will discover you may save 4-5 mpg (indicated average) on an hour's run. Now whether that computed average translates into actual fuel savings is debateable - but at least we know it's a step in the right direction.

 

Today it is $3.05 for regular and I think $3.15 for mid-level and $3.25 for premium (if the pattern is holding). It's up $.02 from yesterday.

 

Put a container of Techron through and fill it up with regular or premium. I filled mine up with regular and it ran excellent...Then I was told to run it on the highway for long time and then let it burn all the way down...This does not mean by driving to the redline every time. Just let that tank deplete itself after driving on the highway atleast once. Then fill up the new tank with the opposite of what you put in it with the Techron. (i.e. reg if u used pre or vise versa). Then after that burns, you should've cleaned out everything having to do with fuel pretty good.

You can now go for your normal gas choice.

Worked great for me...58k miles

 

Did your state remove MTBE and put in 10% Ethanol in your gas? I noticed a drop off after the switch. When I get the good stuff out of state the car runs much better and gets better MPG.

I replaced everything under the sun to up my MPG to what it used to be but in the end it was for not, the new gas is here to stay and lower MPG and hotter engine temps are the norm.

 

I think it's now 10% Ethanol. I don't recall how long it has been that way. I will take me weeks to lower my tank level enough to try the techron and switching to premium for a tank suggestions.

 

In my experience, temperature and A/C usage have little to do with gas mileage. Mine never go up or down more than 1 mpg for my commute.

A 30% drop means your engine is running rich all the time. I suggest that you check the MAF sensor. Or simply take it to the dealership for diagnostics.

 

What gas is recommended for the 1999 rx 300? 87,89, or 91?

 

Inside the rear cover of your owners manual..
(And much debated)

"Select Octane Rating 87 (research octane number 91) or higher.
For improved performance, the use of premium unleaded gasoline with an Octane Rating 91 (research octane number 96) or higher is recommended "

May I suggest you search the forum for additional opinions. The 3.0 engine in the RX is a 10.5 - 1 compression engine.