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Discussion Starter #1
While driving long distances on the interstate I notice that when the cruise is set I slowly fall back from the traffic around me. I will bump + to increase the speed and I will slowly catch back up. I would say I say I catch up 2-3 times as fast as I fall back. Traveling from Cincinnati to the gulf with two cars was pretty annoying. Curious if anyone else has this problem or if any has a fix.

I have replaced the tires with the recommended sizes, not that they were ever different. It seems to me that the speedometer is a fraction off. I have tried using GPS speed apps to compare. Although, it does seem a bit off I have yet to find an app that shows tenths of mph. Perhaps a more scientific approach or switching to Kmh would tell me for sure if the speedometer is not accurate.
 

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Here’s a thought. Cruise keeps your speed steady, usually within 1. When you go up a hill, cruise keeps it steady. Other drivers lose speed until they realize they need to accelerate. You pass them. Opposite going down a hill. Drivers don’t start decelerating until they are going too fast. They pass you.
 

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Discussion Starter #5
Is the car losing speed or are the other cars speeding up?
I’m maintaining speed as normal cruise control does. Other cars seem to be holding their speed or are on their cruise. Going back to my trip to the gulf, my dad driving the other care was certainly using cruise. I don't expect to keep up perfectly with another cars cruise but I do expect it to be close over 10+ miles.
 

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Discussion Starter #6
Here’s a thought. Cruise keeps your speed steady, usually within 1. When you go up a hill, cruise keeps it steady. Other drivers lose speed until they realize they need to accelerate. You pass them. Opposite going down a hill. Drivers don’t start decelerating until they are going too fast. They pass you.
Thanks for the reply. Totally considered this and completely going to happen if others are not on cruise.
most of the time I get on the highway or interstate and match speed with someone and set the cruise. It does take long to find out if others are on their cruise or very good about hold speed. Either way I slowly fall back of begin gaining on them while holding speed with my cruise. My cruise is just not close enough to my liking to most other cars on the road. 65 mph maybe 64.75 in my car or 65.75.
 

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Cruise setpoint changes based on how many and how long you hold the 'increase' or 'decrease' button. It's somewhere near 1mph per tap but that's a guess by observation not a known fact. So rather than using that, cancel and set the speed setpoint again after you've determined that you are going exactly the same level-ground speed as the "traffic" you are in. Note the reading on the speedometer. Now just drive with feet off the pedals an fingers off the increase and decrease buttons. Watch the speedometer. As you find yourself "falling back" from the reference traffic on level ground, look to see how much the speedometer reading has drifted from what it showed. Did your speed change from what you set it at?

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The speed control takes a snapshot of the speed the car is travelling when you push the "set" button. When it detects a speed lower or higher, it adjusts throttle opening to get the car back to the set speed. The response of the control is a balance between how big the difference is and how long the difference has existed, plus whether the error is getting bigger or smaller and how fast that error is changing. For passenger comfort, the response is dampened so you don't get carsick. The car starts up a grade or you encounter a headwind, the car slows slightly until the control detects the speed error and adds throttle gradually. Once the change in error stops and reverses, no more throttle is added so long as speed is correcting. As error approaches zero (speed = setpoint) the throttle is adjusted towards closed so actual speed doesn't overshoot the setpoint speed. All this plays together to say that so long as there are no external varying factors (incline or decline, wind or drafting effects from traffic around you) the control will hold target speed perfectly. The system only makes throttle changes when it's no longer perfect, and the changes are slow enough to avoid bothering passengers.

Want to see a great show? Pull up the fuel economy display, an effective look into the load placed on the engine. In cruise mode, it gives you a great indication of how much the workload on the engine changes even when the car seems to be going at a constant speed. It's a pretty impressive display of how well the system makes adjustments, usually at levels and with comfort that are not perceptible to our own personal inertial guidance systems.

Read more about the control algorithms by researching "PID control".
 
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