1998 LS400 Highway MPG: 75mph vs 65mph - Club Lexus Forums

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1998 LS400 Highway MPG: 75mph vs 65mph

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Old 03-06-13, 04:45 PM   #1
ls400geek
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Default 1998 LS400 Highway MPG: 75mph vs 65mph

Yesterday we did a 500 mile round trip on some fairly flat road.

On the way down there, we filled up at a gas station right at the freeway. We drove at a maximum speed of 65mph (10 less than speed limit for a long stretch). Refilled again at another gas station right off the freeway and we got 27.5 mpg.

On the way back, again only stopping at gas stations close to the freeway, we drove a maximum speed of 75mph (the posted speed limit for a long stretch). We calculated 25.5 mpg.

So it appears we saved 2 mpg by going 65 instead of 75.

Just thought someone might find this interesting so I posted it.
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Old 03-06-13, 05:11 PM   #2
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Did you have any time savings when you drove faster? Was it worth it?
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Old 03-06-13, 06:35 PM   #3
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Was that with cruise control engaged? Also curious about the time difference
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Old 03-06-13, 07:12 PM   #4
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Thanks for the report. You now have a good gas mileage! Your efforts are showing good results.

Although the best fuel efficiency rpm is closer to 75mph but the more you run fast, the more you get the air resistance and your figures may be natural.

Come to think of time and money issue, you spent about 30 minutes more running 65m/H and saved 0.7 Gallon. I don't know the current gas price there but here in Japan that costs about 430 Yen or 4.7 Dollars.

The minimum wage set by the law here is about 9 Dollars/H and the time you have saved is almost the same as the minimum wage rate in Japan. Due to my engineering skill I get some more money than this, but I prefer to run not fast for the less stress and the safety reasons.
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Old 03-06-13, 07:15 PM   #5
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How about the wind direction and speed? Were you going against the wind when driving 75 mph?
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Old 03-06-13, 07:22 PM   #6
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Ive made numerous 1200mile round trips from IL to NC

I find no matter how fast I 65-100... As long as you accelerate gradually you will yield at least 25-27mpg.

Approaching inclines I tend to accelerate to 80-90 and let off the throttle some and let the mph drop off to the speed limit some times. By the time I reach the peak of the incline I'm going 5 under the speed limit or 5 over.

Its a 9.5-10hr trip one direction. I've made it in 8hrs before. 24mpg was the average.

lately I've been making the trip in 9hrs. with the average of 25-26mpg.

I've made the trip accelerating gradually and averaged 70mph and got 28MPG.

The time savings is worth the speeding And more $ spent on gas. More enjoyable drive with the ocassional WOT.

In metro area and areas where the speed limit drops from 70 to 60 or 65 I usually went 5 over. Some metros i cruised the speed limit. I do not drive with a radar detector. I scan the highway and look for spots where a trooper would be able to park hidden.

Out of the 18,000 miles in the past 2 years Ive seen 6 cars that at one point was in a "pack" with me get pulled over with in 30mins to a hour after they left the pack.
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Old 03-07-13, 04:43 AM   #7
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Quote:
Originally Posted by mr. tetsuo View Post
Was that with cruise control engaged? Also curious about the time difference
The cruise control was engaged. And of course it takes a little longer when you drive 10 mph slower But my time isn't worth all that much.
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Old 03-07-13, 04:45 AM   #8
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Quote:
Originally Posted by vindell View Post
How about the wind direction and speed? Were you going against the wind when driving 75 mph?
Hrm I'm not sure. I don't think it was that windy.
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Old 03-07-13, 05:32 AM   #9
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Sounds about right. Air resistance is the velocity squared so a small increment in speed is a big increment in air resistance. So between 65 and 75 you get a 33% or so increase in air resistance. Depending on what percentage of the total resistance that the car has in forward motion is air resistance depends how much it will it will affect your car. I suppose on a truck with high air coefficient the effect will be a much larger percentage on the gas mileage.
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Old 03-08-13, 11:56 AM   #10
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I drove 1000 miles yesterday and averaged 26.7 over the first 500 and 27.8 over the second 500 miles. Both trips on 93 octance fuel average speed 70-80 no air conditioner on the second 500 miles. I'm confident I could do 29-30 m.p.g. strictly highway miles on a short trip.
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Old 03-08-13, 05:31 PM   #11
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I did a 70 mile short trip recently, filed it up before and after, and actually got 30 to the gallon out of it. I am taking her to California next week, and will see what she pulls on the road and in the Colorado Mountains.....
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Old 03-08-13, 05:50 PM   #12
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lucky you 98 up guys get a bit better MPG due to having an extra gear! i bested a 25MPG on my 97 on a 300mile roadtrip going between 75-85.
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Old 03-08-13, 11:32 PM   #13
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Quote:
Although the best fuel efficiency rpm is closer to 75mph
I question this - because most modern cars get the best mileage at the lowest rpm that they will run in the overdrive gear, and in my LS400, this is around 38 mph, so with wind resistance in my favor, I submit that this is likely the most efficient speed and the most efficient rpm would be the associated 1600 rpm or so.
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Old 03-09-13, 05:04 AM   #14
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The performance curves attached may help you to understand why.
75mph is about 121km/H and this is closer to the best fuel efficiency rpm=2400rpm.

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Old 03-09-13, 06:47 AM   #15
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Quote:
Originally Posted by Yamae View Post
The performance curves attached may help you to understand why.
75mph is about 121km/H and this is closer to the best fuel efficiency rpm=2400rpm.

You are ignoring wind resistance. My guess the optimum is 60mph

http://www.mpgforspeed.com/

http://physics.ucsd.edu/do-the-math/...g-on-gasoline/

Last edited by sam12345; 03-09-13 at 07:20 AM.
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