Rx400H Mpg Analysis

[cduluk]

Hey guys,

I've had my Rx400h for about 2 years now and thought i knew it very well, but i just discovered something very interesting today and thought i'd share it with you.

My MPG averages have always been in the 20MPG-23MPG range, which i always thought was a bit "low" for the Rx400h considering i use 91-93 octane. I also noticed that in the summer months i would be getting around 23mpg while in the winter months, much less, usually around 20mpg.

As i'm a college student, about 90% of my driving is spent going to and from school, which is about 7 miles away in either direction, on roads where the speed limit is ~35mph (but of course i probably average 45mph...). And since the parking at my school is so scarce (and i'm incredibly picky about where i park), i make sure to arrive about 30-40mins early each day. When i get there, i usually find a corner of a parking lot and sit with the car "ON", waiting for someone to leave so i can take their spot. It might sound strange, but it gives me a good opportunity to catch up on homework and what not before class, while ensuring i won't be parked "next to" anyone else. I LOVE parallel parking- no door dings!

In any event, last night about half-way home, i pressed the "reset" button for the MPG counter, for no apparent reason. When i got to school today, much to my surprise, this is what it read! I know this was only with 10miles worth of data, but it represents the path i take everyday.

I couldn't believe it! I have NEVER seen this good of mileage with this car! This was taken at 2:21pm right when i got to school. So i started doing some school work, and looked up at 2:27pm and noticed my mileage had dropped:

Then at 2:41pm it had dropped some more:

And before you know it, at 2:56pm it had dropped to 19.7mpg:

I'm guessing this "extreme" drop is due to the fact that i had "reset" the system only 10 miles previous to this event. So while the car's computer was calculating the "miles/gallon" average, the numerator remained at a constant 10 miles while the denominator (the amt of fuel the car was burning sitting in park) was increasing, resulting in a lower mpg number.

I'm sure if i had reset the system 200 miles before today v.s. only 10 miles before, the small amounts of fuel i'd be burning "sitting in park" wouldn't affect the mpg number all that much. But still, now i know why my MPG averages have been so bad!

And i also know why in the "summer months" my mileage has been so much better- not as much school! AKA less time in idle.

Just thought you guys would enjoy my findings! It was amazing watching the MPG number decrease by 1/10ths. The engine would turn on for ~8 minutes then go off for a while, and it was when the engine was running that the mpg number would decrease. I took a picture each time the number went down during these times, and there were a few times when i didn't have enough time to reset my camera before the number would go down again! It was very interesting...

[katzjamr]

cduluk, our group is a much better place because of your posts.

[lemon]

The idling drops your mileage dramatically. I remember you posted a while back about the effect of idling while you waited for a parking spot at school. Someone responded that the minimum you would be doing is worsening your mileage. Now you see it right in front of your eyes. Also, the mileage is based upon number of miles driven, which is why it was so good when you first reset the computer.

You will see it change as more miles are put on. I remember once resetting the mileage while in a very hilly area. As I coasted down the hills, I reached as low as 4 or 5 litres/100km. If I did this after 3000km worth of data, the mileage change on the computer would have been much less.

[SW03ES]

It really does have a huge impact on your mileage. We have a remote start on the Jeep and it sits outside. My wife used it a few times when it got cold (its been fricking 70 degrees lately) and the mileage hit just from it idling for 5 minutes before she got in is HUGE. So much so that I doubt we'll continue to use the remote start.

[lemon]

70 degrees??!! That's frickin' cold? LOL. Shorts and t-shirt where I live. Yes, idling is returning 0 MPG

How do you like the Jeep? I really like the new design. I had a 1998 5.9 Limited...wasn't the best on reliability but, my God, the motor...talk about torque..

[maseace]

I reset the MPG screen at each fill-up. The best I have gotten on a whole tank is 32-33 mpg on a drive down the Oregon coast in the summer (55 mph speed limit).

You should try the waiting for parking thing with the car OFF (or accessory) and compare mpg to when you let it idle in 'ready' mode.

[SW03ES]

How do you like the Jeep? I really like the new design. I had a 1998 5.9 Limited...wasn't the best on reliability but, my God, the motor...talk about torque..

We love it, really well built, drives great...looking forward to some snow.

[nlayla]

Hi,

Please reply with additional information would be greatly appreciated! This is in relation to the mpg drop on my 2008 RX400h AWD. I used to get 28mpg and now I'm currently getting about 20.2mpg. What is going on? Could it be the cold weather? Where I live, we are getting 30-55 degree days and during the summer it is about 85 degrees. But truly, the change started happening after the IPM recall replacement in September 2013, I noticed a huge drop in my MPG! The MPG isn't as efficient as it was when I first bought it in August 2013. I used to get 25mph-28mph and that is very efficient! I've reached out to the Lexus dealership who replaced the IPM multiple times and they keep reassuring me that it's reading normally and that's how it is supposed to read! You see, I drive my car every day and I know how it runs and it isn't the same anymore after the IPM replacement. I also notice that the gas engine pops in more frequently at a complete stop while waiting at a red light. I also notice that when I'm barely accelerating (less than 10 mph), the engine pops in too. Whereas before, the engine doesn't kick in until I accelerate to about 25mph, it would used to be all silent hybrid engine until it reaches 25mpg. I like how my car ran before the IPM recall replacement. I take good care of my car, it gets serviced at the Lexus dealership whenever service is needed. All services are up to date with my car, I just don't know what's going on. Any pointers as to how to get MPH back up and more efficiently and why the MPH has dropped so significantly...would be great! Thank You in advance.

-nlayla

[maseace]

Make sure they did the ISC reset (idle speed control). Many places forget to do this after disconnecting the battery.

[nlayla]

Thank you so much! You're the best! I will ask that the Lexus dealership to look into this. I do have a feeling that it wasn't reset. I'll be back with an update!

[RX400h]

Keep in mind that mileage drops during colder months (yes, even here in San Diego). This is true of any vehicle - hybrids and non-hybrids. I'm sure you've, on occasion, turned on the seat heater(s) on cold mornings, especially if you're not parking your RX in a garage.

[maseace]

Tips for better MPG in the RX400h:
- Keep your tire pressure at 38-40 PSI.
- Get low rolling resistance tires (LRR) if you don't have them. The OEM Michelin S8 tires are LRR (Green-X). These will add about 3 MPG to non-LRR tires.
- Turn off the A/C if possible (summer AND winter, in AUTO mode the AC is on by default at any set temp). Go to the climate screen and disable AC, and you can still keep it in AUTO without running the A/C compressor.
- Use the heat sparingly in winter. Having a high set temp will make the engine run longer, especially until the engine is warmed up (157° F and up). The heated seats can compensate for a lower cabin temp.
- Keep it under 42 mph to allow longer gliding on battery only. Feather the gas to keep it around 40 mph. Works great when the speed limit is 35 or 40. I can sometimes get up to 1-2 miles on battery only.
- A highway speed of 50-60 mph will get you MUCH better MPG than 60-80 mph. My experience in summer: 55 mph - 32 mpg, 75 mph - 26 mpg.
- Don't accelerate hard, coast as much as possible, and brake lightly to use regenerative braking instead of the friction brakes.
- Buy a ScanGauge computer so you can see the engine RPM and engine temp, so you can practice keeping it in battery only stealth mode at 0 rpm.
- Make sure to perform the ISC reset any time the 12V battery is disconnected for service or gets low on charge. The effectiveness of this is debatable but it can't hurt.

[nlayla]

Hi everyone, thank you for your inputs. Mason, thank you very much for the tips for better gas mileage. I'll definitely do that. Dave, thanks for the reminder about the cold weather, it is true. I have learned a lot about the hybrid engine these past few days. After further research and observations and your inputs, it is in deed the cold weather that is causing the slight drop in mpg. What's happening is that the hybrid engine takes longer to warm up during the cold winter which means the gas engine will pop in more frequently because it is working harder to warm up. And, the reason why I'm getting 20.3 mpg is because I only drive less than 5 miles to work which doesn't give the hybrid engine enough time to warm up. Additionally, I reset the trip odometer after every gas tank fill up and the initial few miles may show that I am only getting 20.3 mpg because it doesn't have enough data to calculate. However, over the weekend, I did some "good solid driving" and my mileage went back up to about 24.5 mpg. I am happy. And, I also learn that the the 2008 models do not need to get an Idle Speed Control reset as it automatically calibrate itself. This is the first winter with my car and I am learning a lot about it. I really do enjoy my car. Love it!

[RX400h]

Layla,

How often do you have your oil changed? If you do NOT take longer trips, say, every weekend, your oil may not have a chance to get rid of moisture which is more prevalent during Winter months. Of course, the only way to tell with certainty whether the moisture content is excessive at current oil change intervals is to have a sample of that oil, tested. Blackstone labs is a major player in the oil-testing field.

If you're using non-synthetic oil, I would recommend that change intervals not exceed 5 months.

Dave

[nlayla]

Hi Dave :),

I will definitely reach out to Blackstone labs for oil testing. I'm so glad to learn about the moisture level. I get oil changed as recommended by the dealership's sticker or when the maintenance light comes on. As far as oil type, that's a good question. I sure hope that the Lexus dealership uses the best kind of oil. Thanks so much for the invaluable information to a healthy car. I'll keep everyone updated from time to time. Hope everyone is well. And, Happy New Year!

Best,

Layla