Solving the mpg mystery on my 2008 RX400h
09-03-11, 10:47 AM
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Solving the mpg mystery on my 2008 RX400h
I bought a CPO 2008 RX400h AWD with 25k miles on it a month ago. I am new to Lexus and hybrids, but have done my research. I love the car, if I could just figure out what is happening with my mpg.
I do mostly short runs in "city" traffic, which is wide suburban roads with mostly 45 mph limits, lots of red lights, and some neighborhood streets at 30 mph with stop signs. There are some hills, not huge, but enough to have the car show me 6 mpg when starting up it. Traffic flows pretty fast from light to light, it's usually not backed up. The past month has also been like 100F, so the airconditioning runs high and non-stop. I verify my mpg numbers from the car computer each time I refill the tank. I started getting 20-22 mpg in the 'city', driving like a granny, and obviously wasn't impressed. I did the ISC reset and used pulse driving. I got up to 28 mpg for like 4 miles, then it slowly got worse and worse. So I started religiously tracking what the car is doing. Here are my observations:
1) When I did the ISC reset, the battery charge indicator never moved, despite the engine reving. (A/C, radio were off.)
2) No matter how and where I drive, the first 5 min (Consumption display under Info) are always below 20 mpg, sometimes as low 15 mgp. The engine runs all the time when I am moving, no matter how gently I coax it.
3) The display then becomes like stairs, each progressive 5-min interval gets better, topping out somewhere around 35 mgp if I try to maximize the hybrid technology. But since I seldom drive for more than 15 or 20 min, the law of the averages wins and I don't get over 24-25 mpg.
4) The battery indicator never shows 'full'. The highest it has gone is level with the -+ signs on the display.
5) Recently I drove all highway for 35 miles, same weather, and got 28 mpg! I averaged 28.6 mpg with avg speed of about 65 mph, reduced to 28 mpg with avg. speed closer to 75 mph.
Some other important facts
- I am using Premium gas, but have only gone through 3 tank-fulls in the past month. I'd like to switch to regular.
- I work from home, so sometime the car sits in the garage for couple of days without being driven.
- The car was leased for 2 years, then auctioned off at the end of May. My Lexus dealer had it on the lot 2 months (summer temps) before I bought it.
I apologize for this long message, but I wanted to share all my info that ultimately makes a difference. Here are my questions:
Is it possible I didn't do the ISC reset correctly or it got "lost"? How can I tell? What makes it work one time and not the other? (I am too much of an engineer to see this as a black box
)
Could I have a marginal battery? How can I test it? What about the auxiliary one?
Is there anything I can do to eliminate that "first 5 min" mpg gas-guzzling consumption?
Why am I getting better highway mileage than city?
I do mostly short runs in "city" traffic, which is wide suburban roads with mostly 45 mph limits, lots of red lights, and some neighborhood streets at 30 mph with stop signs. There are some hills, not huge, but enough to have the car show me 6 mpg when starting up it. Traffic flows pretty fast from light to light, it's usually not backed up. The past month has also been like 100F, so the airconditioning runs high and non-stop. I verify my mpg numbers from the car computer each time I refill the tank. I started getting 20-22 mpg in the 'city', driving like a granny, and obviously wasn't impressed. I did the ISC reset and used pulse driving. I got up to 28 mpg for like 4 miles, then it slowly got worse and worse. So I started religiously tracking what the car is doing. Here are my observations:
1) When I did the ISC reset, the battery charge indicator never moved, despite the engine reving. (A/C, radio were off.)
2) No matter how and where I drive, the first 5 min (Consumption display under Info) are always below 20 mpg, sometimes as low 15 mgp. The engine runs all the time when I am moving, no matter how gently I coax it.
3) The display then becomes like stairs, each progressive 5-min interval gets better, topping out somewhere around 35 mgp if I try to maximize the hybrid technology. But since I seldom drive for more than 15 or 20 min, the law of the averages wins and I don't get over 24-25 mpg.
4) The battery indicator never shows 'full'. The highest it has gone is level with the -+ signs on the display.
5) Recently I drove all highway for 35 miles, same weather, and got 28 mpg! I averaged 28.6 mpg with avg speed of about 65 mph, reduced to 28 mpg with avg. speed closer to 75 mph.
Some other important facts
- I am using Premium gas, but have only gone through 3 tank-fulls in the past month. I'd like to switch to regular.
- I work from home, so sometime the car sits in the garage for couple of days without being driven.
- The car was leased for 2 years, then auctioned off at the end of May. My Lexus dealer had it on the lot 2 months (summer temps) before I bought it.
I apologize for this long message, but I wanted to share all my info that ultimately makes a difference. Here are my questions:
Is it possible I didn't do the ISC reset correctly or it got "lost"? How can I tell? What makes it work one time and not the other? (I am too much of an engineer to see this as a black box
)Could I have a marginal battery? How can I test it? What about the auxiliary one?
Is there anything I can do to eliminate that "first 5 min" mpg gas-guzzling consumption?
Why am I getting better highway mileage than city?
Last edited by 2xNewbie; 09-03-11 at 11:01 AM. Reason: Typos
09-03-11, 11:11 AM
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My 08 RX400h is only getting about 24 mpg in city driving. I've only had it for about a month, so I don't have much long term data. However, I have noticed that the really hot weather we are having seems to kill the mileage since the A/C runs a lot.
Hopefully with cooler weather the mileage will improve.
Jeff
Hopefully with cooler weather the mileage will improve.
Jeff
09-03-11, 01:31 PM
#3
Pole Position
You know I am in a hot climate (Sacramento valley) and ours gets on average about 24 to 25....Highway it get about the same....I have done the ISC reset also....best mileage was 26mpg....My wife does mostly city commute and that seems to be what it gets...The other factor is your gas have higher than 10% ethanol blend...I know the gas in california sucks, as when I go to Nevada the mileage is always higher on their gas...California formulates their gas for lowest emissions. As for running regular, that is a long discussed thing on all of the lexus cars, I personally run premium and once in a while will put mid grade in, but very infrequently...I know the engine computer compensates but the recommend fuel is premium so that is what it gets.
09-04-11, 07:20 PM
#5
Registered User
Mileage
My 2007 RX350 FWD is driven 1/2 city and 1/2 highway. And my display shows I me getting around 24 miles to the gallon.
But that is high. When I do the actual gas to miles. By checking my trip meter to the amount of gas I put in. It averages out to almost 19.
Have you checked how much gas you put in to actual miles you have driven?
But that is high. When I do the actual gas to miles. By checking my trip meter to the amount of gas I put in. It averages out to almost 19.
Have you checked how much gas you put in to actual miles you have driven?
09-05-11, 04:54 AM
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Our 2008 RX400h consistently gets 26-27.5 mpg according to the display with 89 octane gas in all suburban / grocery getting driving. Some suggestions to consider are:
-- feathering the gas pedal at stops so that the battery takes you up to 5mph
-- get to driving speed promptly and then let the battery hold your speed at the 35-40 mph level
Hope this helps!
-- feathering the gas pedal at stops so that the battery takes you up to 5mph
-- get to driving speed promptly and then let the battery hold your speed at the 35-40 mph level
Hope this helps!
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