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Discussion Starter · #1 ·
I was just on the highway and I LOVE the way the Volt's cruise control works. Once you have it set a little window pops up on the display that tells you at what speed the cruise is set at....and the vehicle very accurately keeps the speed you have it set to. Once the cruise is set you can hit the toggle button on the steering wheel up once and the cruise will increase by 1 mph. Likewise you can hit the toggle down and it will decrease the cruise speed 1 mph. Each time the indicator box will pop up telling you the speed that it is set at.

I was just traveling on the highway and I set the cruise to 65mph. The little box popped up and said it was at 65mph. I hit the toggle up once and the box popped up and said it was still set to 65mph...but the speed increased to 66mph. I hit it up once more and the box said it was now set to 67mph and the speed increased to 67mph.

I tried going down and hit the toggle and the box said I dropped to 65mph but the car was going 66mph. I hit the down toggle again and the indicator box again said 65mph while the speed dropped down to 65mph.

So basically the indicator box will not say that the cruise is set to 66mph. It seems to be programmed wrong to say 65 when it is really 66. I toggled down to 60mph and found the same problem with 62mph. It would only say 61.

Basically I would think it is just a typo that needs to be corrected in the software. Anyone having the same issue?

I guess with the millions of things that could go wrong with this highly advanced vehicle, a little typo isn't so bad! :cool:
 

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Do you know if they can send updates to the software via OnStar or do you have to take it to the dealer? Good find.

Matt
 

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Discussion Starter · #3 ·
Hi Matt.

I really don't know. I would guess it is a download at the dealership...only because I remember reading an earlier thread talking about updates that GM had been doing early on and I think they said the cars had to go to the dealer to get it.
 

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The cruise control in both my Civic and Enclave work the same way.

Each time you tap the button up or down, the speed changes by approximately 1 MPH. Keep in mind you're looking at a digital speed display that is using integer values, (no tenths), so it is rounding up or down. In other words, if you’re going 66.4 MPH and tap the CC button to slow down, and Volt slows 9/10 of a MPH to 65.5 MPH, the speedometer will still display 66 MPH – even though in reality you’ve slowed approximately 1 MPH. I've noticed this on my Enclave every now and then also, which also gives a digital read out of the cruise control speed setting in the information display window when you set or change it. I don't know the exact accuracy of GM's digital cruise control, but it will fluctuate around the desired set speed on level ground by a few tenths of a MPH due to perturbations in the environment (wind, incline, road surface, etc.)…and more if you’re driving up/down a steep incline

BTW, if you hold down the toggle button, rather than tap it, I'm guessing the Volt will continue to accelerate (or decelerate) until you release the button. That's standard cruise control operation on most modern cars.

Cheers
 

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Discussion Starter · #5 ·
I think Rooster may be right.

Maybe when I had set the cruise to 65 it was really at 65.5 mph (but it only displays whole numbers) so every few clicks up or down it would round the speed the wrong way. I went out and tried it again and this time every toggle seemed to be exactly 1mph up and down so I must have been closer to the actual speed indicated.

I use the cruise a lot and this was the first time I had seen that phenomenon. Of course as Rooster says you can also hold up or down the toggle to change the speed to the point that you let go of the toggle. Thanks for your help!
 

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Maybe when I had set the cruise to 65 it was really at 65.5 mph
Is the speedometer based on wheel rotations (as is typical) or on the GPS (which can be MUCH more accurate)? Does the car double check for sanity between the two? If I put 16" snow tires on (OK, not likely for me) will the car figure it out based on the descrepency and adapt?

It should.
 

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Is the speedometer based on wheel rotations (as is typical) or on the GPS (which can be MUCH more accurate)? Does the car double check for sanity between the two? If I put 16" snow tires on (OK, not likely for me) will the car figure it out based on the descrepency and adapt?

It should.
In a word. No.
The Volt's road speed is derived from the transaxle output shaft speed sensor.
So replacement tires should be kept to as close as factory circumference as possible

To the OP, as Rooster suggests there's a tolerance applied to all of the speed values you've discussed.
In a nutshell there's nothing wrong with our Volt.
The tap-up tap-down speed change of 1mph is merely an approximation
WOT
 

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Rooster is right and Michigan figured it out. It depends what speed you are going when you set the CC. If you are close to the .5 it may not change the number or increment 2 MPH when you tap the up/down control. Corvettes have had this feature since 1997 and they behave the same way. You may also notice the “Cruise Set at” number may be about one MPH higher than the speedometer displays. This is normal as well.

While it is true that GPS is more accurate while traveling at a steady speed because it is not affected by tire circumference variation, GPS will not be as accurate during acceleration, deceleration, and rapidly changing grades up or down. This is because the typical automobile navigation GPS only updates about one a second so the displayed speed can lag the actual speed. There are high speed GPS units that update 20 or more times a second and provide accurate near-real-time speed, but they are still quite expensive and limited to racing data acquisition applications.
It would be possible and very cool, if the on-board computer compared the GPS speed at steady state to the speedometer and automatically tuned the speed calculation from the wheel speed sensors for variation in tire circumference. That would provide the steady state accuracy of GPS combined with the real-time rate-of-change accuracy of wheel speed sensors for no additional cost.
 

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In a word. No.
The Volt's road speed is derived from the transaxle output shaft speed sensor.
So replacement tires should be kept to as close as factory circumference as possible

To the OP, as Rooster suggests there's a tolerance applied to all of the speed values you've discussed.
In a nutshell there's nothing wrong with our Volt.
The tap-up tap-down speed change of 1mph is merely an approximation
WOT
Thanks for the feedback because I have seen, and wondered about, the same issue. I guess it is because I have never had a car with this level of granularity in the speed control department that I am seeing this "problem" for the first time. Thanks to all who share their experience and knowledge....
 
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