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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.
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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.
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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)

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