Significant difference in IS350 fuel economy between 60 and 70 MPH
06-09-08, 08:35 AM
#1
Significant difference in IS350 fuel economy between 60 and 70 MPH
Have you guys noticed the dramatic reduction in fuel economy between 60 and 70 MPH? I regularly average 35+ MPG driving to / from my boat on weekends by setting the cruise control at exactly 60 MPH (via GPS). However, this weekend I decided I would do the best I could to maintain 70 MPH (in snow mode) to see what I could get. I couldn't use cruise control due to traffic, but I was able to keep the speed pretty damn close to 70 MPH without slowing down too much. Whenever I did have to slow down, I would accellerate very slowly back to 70 again. To my extreme surprise, I couldn't even get 29 MPG! That's almost a 20% reduction! I did the return trip at 60 MPH just to make sure there wasn't something wrong, and I averaged over 35 MPG just as usual.
I wonder why this is. I know that aerodynamic drag increases exponentially with speed, but that still doesn't quite justify it in my mind. I wonder if there is something "magic" going on at 60 MPH that makes the engine run much more efficiently (intake / exhaust harmonics?).
I wonder why this is. I know that aerodynamic drag increases exponentially with speed, but that still doesn't quite justify it in my mind. I wonder if there is something "magic" going on at 60 MPH that makes the engine run much more efficiently (intake / exhaust harmonics?).
06-09-08, 08:55 AM
#2
Lexus Fanatic
Snow mode retards the timing and actually hurts fuel economy which you would not expect. The manual states this and only advises using either power or snow mode when needed in order to obtain the best fuel economy with the standard mode. Higher speeds typically always yield higher fuel consumption.
06-09-08, 09:06 AM
#3
^^^ If the manual says that Snow mode retards timing and is less efficient, then it's wrong. I've done lots of testing to shows that the only differrence to the engine between the modes is throttle sensitivity. At full throttle, the engine performs identically in all three modes. It has also been well documented on this site that snow mode gives better fuel economy in stop and go traffic. However, I forgot to mention that snow mode is always used when I do my 60 MPH tests.
06-09-08, 09:08 AM
#4
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Snow mode basically controls gas pedal & throttle sensitivity / input.
60 mph puts you just at or below 2,000 rpms...
On my way back from the track Saturday, I filled her up and set the cruise @ 60 mph.. I think I hit 32 mpg just before I got off the freeway, 30 minutes later... (regular mod)
^^ This was after a long day of racing in the heat and burning up lots of fuel.. LoL...
If traffic permits I try to stay in the 65-70 range here on Los Angeles Fwy's... Any slower and you just might get shot...
60 mph puts you just at or below 2,000 rpms...
On my way back from the track Saturday, I filled her up and set the cruise @ 60 mph.. I think I hit 32 mpg just before I got off the freeway, 30 minutes later... (regular mod)
^^ This was after a long day of racing in the heat and burning up lots of fuel.. LoL...
If traffic permits I try to stay in the 65-70 range here on Los Angeles Fwy's... Any slower and you just might get shot...
Last edited by Joe Z; 06-09-08 at 09:12 AM.
06-09-08, 09:08 AM
#5
Well for me, at 60mph, the engine is hovering around 1.9k rpm
and 70mph, the engine is hovering around 2.3k rpm.
Maybe that helps a bit?
Also from my experience, noticeable torque for acceleration doesnt come until over 2k rpm. Now how come engine need to spin above a certain rpm to generate torque, why torque curves arent always linear, could be the explanation for this as to why sub 2k rpm can generate over 20% efficiency compare to something a tad over 2k rpm.
and 70mph, the engine is hovering around 2.3k rpm.
Maybe that helps a bit?
Also from my experience, noticeable torque for acceleration doesnt come until over 2k rpm. Now how come engine need to spin above a certain rpm to generate torque, why torque curves arent always linear, could be the explanation for this as to why sub 2k rpm can generate over 20% efficiency compare to something a tad over 2k rpm.
Last edited by mikez; 06-09-08 at 09:11 AM.
06-09-08, 09:12 AM
#6
Greg - I noticed this as well. Two years ago on our caravan to Los Angeles to San Francisco for EOS, we drove ~60 mph for the whole trip, and got almost 35 MPG. I also tried this on one of my cross-country drives from Dallas to Los Angeles, and noticed a dramatic difference in fuel efficiency between 60 and 70. At 70 mph I would get around 28-29 MPG. 55-60 MPH is the sweet spot for maximum fuel efficiency on the IS350.
Javier
Javier
06-09-08, 09:13 AM
#7
Lexus Fanatic
^^^ If the manual says that Snow mode retards timing and is less efficient, then it's wrong. I've done lots of testing to shows that the only differrence to the engine between the modes is throttle sensitivity. At full throttle, the engine performs identically in all three modes. It has also been well documented on this site that snow mode gives better fuel economy in stop and go traffic. However, I forgot to mention that snow mode is always used when I do my 60 MPH tests.
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06-09-08, 09:33 AM
#8
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In my experience, Snow mode does more for my fuel economy when I'm in stop and go traffic and not on the highway. That being said, there is no doubt in my mind that snow mode helps fuel economy (even if it just helps you be more conservative by limiting throttle response). All of the major media outlets have come out with their "Fuel Saving" tips and almost all of them say slowing down to 60 on the highways is key. However on a trip from San Antonio to Dallas last summer, I averaged 35 MPG with an avg speed of 70 and PWR mode enabled the whole trip... go figure!
from vegas to LA after fill up
06-09-08, 10:43 AM
#14
Now that I think about it some more, this could have been influenced by wind speed / direction. I've done this test many times at 60 MPH, and I usually get about 36 MPG on the way to the boat, and 35 on the way back from the boat ~6 hours later. I've never really considered wind as a factor since the results are so repeatable. However, yesterday was particularly windy on the lake, and I would guess that I was heading right into it when I did the 70 MPH test.
