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Old 12-20-2019, 01:46 PM   #1 (permalink)
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Cruise control using higher RPMs with CVT

I have a 2019 kia forte with 2.0 and CVT. I recently tried using cruise control on the highway at about 60mph. While in cruise control the engine speed was adjusted to about 400rpm higher than where it was when I was manually controlling the engine speed. I figured this could be because the computer knows this engine speed will yield greater fuel economy. Unfortunately, after testing the theory several times, this is not the case. I have a 10 mile commute home from work on a stretch of highway that has barely any traffic. On board computer, scanguage, and torque pro app all report same. Im going to be taking a 1000 mile trip soon and would like to use cruise control but not at the expense of fuel economy.
Anyone else experience this? Have thoughts?

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Old 12-20-2019, 01:53 PM   #2 (permalink)
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It depends on the car. In my Insight, I suspect that cruise control would cost me 10-20 mpg on trips (normally gets 80-100+ mpg, so 10-20 MPG loss isn't that much). However, on my mom's Prius I find that the cruise control actually improves fuel economy by a decent amount over driving without it.

I suspect that most modern CVTs fall somewhere in between. Part of the reason cruise control reduces fuel economy on most cars is because it over-revs up hills, only to ride the brakes down the hill. You don't lose economy when you accelerate, rather you lose it when you brake and turn your car's kinetic energy into brake dust.

When I had my Civic, I used cruise control on flat terrain, but turned it off on hills. If you find that there is still a fuel economy loss on level ground, try using cruise control "conservatively" - turn it off for a while, and then turn it back on for a while when your foot gets tired.

I'm surprised that the CVT runs at higher RPMs on cruise control. Is this on level ground? Cruise control on a CVT will constantly adjust the RPMs, supposedly for maximum efficiency. On hills this causes the RPMs to rise, when in a standard car you would keep it in overdrive and slightly lose speed, or do a complete downshift to maintain it.
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Old 12-21-2019, 10:43 AM   #3 (permalink)
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It sounds to me that the Kia's computer is not programmed for maximum fuel economy. That is strange.
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Old 12-21-2019, 12:28 PM   #4 (permalink)
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That is quite odd. From my experience the Mitsubishi Mirage keeps rpms lower than foot control, and the result is superior fuel economy.
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Old 12-22-2019, 10:46 AM   #5 (permalink)
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In my work van (6 speed auto), cruise control almost always keeps the torque converter locked, whereas using my foot, the torque converter is often not. However, it will not always shift into a higher gear by itself. I can use paddle shifters to push it into 6th at 39mph while it won't automatically shift to 6th under most conditions until 45-48mph.
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Old 12-22-2019, 11:21 AM   #6 (permalink)
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I have a similar experience with automatics with cruise control. My previous Saturn L with the 2.2L four cylinder would not lock the converter at 60 mph with the cruise control on. The engine would run at 2600 rpm at 60 and cruise. Turn off the cruise and the converter would lock and the rpm would drop to 2300 rpm. This really affected the fuel mileage at that speed. On my commute using the cruise fuel mileage would drop from 30 to 28.
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Old 03-13-2020, 05:56 PM   #7 (permalink)
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I'm driving Honda Civic with a smaller engine (1,5 liter) with CVT and made a similar experiences.
Eventhough the RPMs are not significantly higher on cruise control, the mileage is irreducibly better if you're not using cruise control.
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Old 03-14-2020, 09:28 AM   #8 (permalink)
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They don't want the car to feel un-sporty.

This guy in another forum once used cruise in 4th gear, so that he would still be in the turbo- he hated feeling the lack of power... in cruise.
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Transmission type Efficiency
Manual neutral engine off.100% @MPG <----- Fun Fact.
Manual 1:1 gear ratio .......98%
CVT belt ............................88%
Automatic .........................86%

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Old 03-14-2020, 09:39 AM   #9 (permalink)
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Quote:
Originally Posted by Fat Charlie View Post
They don't want the car to feel un-sporty.

This guy in another forum once used cruise in 4th gear, so that he would still be in the turbo- he hated feeling the lack of power... in cruise.
I had a relative that would leave my Insight in 3rd gear on the highway, and drive at 80+. He would often return it to me with something like 35-40mpg on the dash.
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Old 03-14-2020, 06:43 PM   #10 (permalink)
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Quote:
Originally Posted by Ecky View Post
I had a relative that would leave my Insight in 3rd gear on the highway, and drive at 80+. He would often return it to me with something like 35-40mpg on the dash.
I can remember times, when more people used to drive like that.

At the time I had my driving lessons, which was probably in 2005, the driving instructors already teached us: unless you're going to pass another vehicle or trying to enter a fast lane, then always drive at the lowest RPMs possible. Modern car engines don't need these high RPMs anymore. If the engine starts stuttering, then RPMs are too low, shift down then. And of course don't drive around in first gear. Only use it to drive one car length then immediately go into second gear. Sometimes I see elderly people or people who don't have a lot of practise in driving, going down a whole road being in first gear all the time.

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