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Old 07-10-2016, 11:11 AM   #1 (permalink)
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Speed limiter helps fuel economy?

I have had a couple of Mercedes where the cruise control can be used as a speed limiter, so that you drive normally, controlling speed as you require, but the car will not exceed whatever speed you have pre-set. I rarely used that feature but just recently I started using it consistently and was surprised to find that it seemed to help with fuel economy. Possibly without it I inadvertently exceed the maximum speed at which I wish to travel, but I wouldn't have thought I do that often. The only other explanation I can think of is that driving a little faster down hills, which I often do, actually uses more fuel rather than (as I would expect) saving fuel by using gravity to my advantage. I would find that surprising if it were true, partly because of the logic and partly because I don't encounter large hills in my area.

I should perhaps point out that this was in a car with an automatic transmission in which I was not happy using EOC so my technique was more DWL rather than P&G.

Has anyone else noticed this? Or indeed tried a speed limiter without seeing this effect?

Unfortunately I have just changed cars and the replacement does not have a speed limiter so I am unable to perform tests to verify/quantify the effect.

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Old 07-10-2016, 02:31 PM   #2 (permalink)
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It's helping you to drive more smoothly, which helps mileage. We call this DWL. When I got good at DWL, not only did I get better mileage, but I regularly put my passengers to sleep.

True story: My very first car, a well used 1961 Rambler American with the flathead six cylinder engine, came with a speed limiter. The gas pedal flat on the floor would only open the carburetor throttle valve a little over halfway. That was good for 60 MPH on a level road. I got 30 MPG.

Then I fixed the linkage, used the extra power, and got 24 MPG from then on. Brake lining wear increased also.
The vacuum gauge plus wheel covers helped increase summer 2015 mileage to 38.5 MPG, while summer 2016 mileage was 38.6 MPG without the wheel covers. Drove 33,021 miles 2016-2018 at 35.00 MPG.
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Old 07-11-2016, 08:36 AM   #3 (permalink)
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Well, it strongly suggests that without the limiter, you would generally drive faster. Speed kills economy.

Lead or follow. Either is fine.
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Old 07-11-2016, 04:12 PM   #4 (permalink)
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not always the case

my GTO gets solidly better fuel economy @70-75mph than at 60
as did my Camaro

but both cars were torque monster V8's with Double OverDrive 6 speeds and tall final gear ratios

when my GTO was stock
I routinely got 31-32mpg (with an avg speed of 70mph over a 400 mile drive)
but in order to get that, I had to keep the speed up the whole drive (coming down to 55 and back up, or taking exits, and getting back up to highway speed brought it down quick)

2005 GTO 6.0 V8 (600hp)

1991 Geo Metro 1.0 3/5 base

2010 Prius II (replaced 2001 Tahoe 5.3 V8 April 2014)

1994 F-350 7.3 TurboDiesel (Rollin' Coal)
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