Cruise Control Setting vs Speedometer Reading

Yea, I noticed that also. But figure 1 mph is not that big a deal. Wonder if others have also noticed this. I also noticed that the speed limit sign turns to red when I am 1 MPH below the posed speed limit. That also seems odd, but not really a problem to me.

 

Can only imagine the conversation with the service writer and anyone else getting involved in the converstaion about DRCC mph setting showing as 1 mph less on the speedometer.
Anyone with a Gen 4 notice this behavior?

 

I think it's a buffer to prevent the car from exceeding the set speed in certain cases. I've noticed the cruise control has about a 1mph "play". When starting to come down a hill, or when the headwind changes, or a lot of other factors, I see the speedo match the set speed. If the car defaulted to the actual set speed normally, that play could mean the car would exceed the set speed for periods of time. I figure Toyota is playing it safe(it is Toyota after all) and setting the default cruise speed on the low end of the range. I just always set the cruise to 2-3 mph above my desired real speed(since the speedo itself is 1-2mph fast).


As for the speed limit sign going red, I'm pretty sure that's the result of different rounding systems. I think the speedo rounds the exact speed and the speed limit system doesn't. So say the speed limit is 60. The car is driving 60.2mph. The speedo rounds that to 60, but the speed limit system says everything over 60 turns the indicator red. If the car drops to 59.9999, the speedo stays at 60, but the red sign turns off. If the car increases to 60.50001, then the red sign stays on and the speedo flips to 61.

 

And how steep a downgrade before the speedo goes 2 or 3 mph over the set speed, or 2 or 3 mph under the set speed when climbing?

 

Yes, it is the same thing in my 2021 Prius Prime Limited. If you set it to, say, 70 mph, the speedometer speed will vary between 69–70 mph.

Moreover, the actual speed is 2 mph less than the speedometer speed.

 

I turned mine off. It was an annoying distraction.

 

It's operating the way it should. I believe there is a plus or minus 1-2 mph deviation to set-point. Watch your speedometer next time the roadway dips below grade at a crossing - I'll bet your speedometer will rise up to 71 mph - if it's a large enough dip. If the car is forced to maintain a set speed, exactly, I believe in most cases - you'll burn more fuel. The cars CPU doesn't know what's coming, incline, decline, wet pavement, so a "skate mode" designed to conserve energy. When the car's speed drops down to or at 1.5 mph below set-point, the ECU sends more power to the wheels. If the opposite occurs, you'll see the engine shut-off and the car will drop into charge mode, applies regenerative braking.

IMHO; if it really bothers you that much - set CC to 71 mph to achieve your desired 70 mph. We're not robot and can think for ourselves.

Hope this Helps....

 

That is because some people don't know anything about control theory. They don't realize that the feedback loop works according to the error signal, in this case about ±1 mph. Trying to make this error smaller can cause other problems, worsen fuel economy, etc.



Control theory—Wikipedia

 

Reminds me of steering a ship.

As an ordinary seaman on a DDE (Canadian destroyer escort), part of most every at-sea shift included a stint at steering. My first time , after several tense minutes trying to stay precisely on the bearing, the other guys shared some tips. Mainly, to loosen up, let it wander a bit. If it continues to drift, then nudge it back, give it time to react.

They also gave an analogy: if you were flying over when someone was over-correcting while steering, you'd see the ship following a sawtooth-like course.

 

I haven't experienced the exact issue, but it's worth checking with the dealer. It could be a calibration discrepancy.

 

That is a very good example.

 

Mine operates as OP mentions.

On a side note: on a recent 2400 mile road trip through mountainous areas of CA, AZ, and NM the cruise control speed did vary slightly but only by +/- 1 MPH on the inclines. I was amazed how accurate the system is and how powerful the ICE is.

 

I drive on one stretch of hwy that dips for 1/3 mile with the steepest dip at the bottom followed by a kinda steep uphill for another 1/2 mile. If I set cruise at anywhere between 55 and 60 the cruise has to hold speed going down and always losses 2 or 3 mph going up the other side. If I set cruise at 62 mph or above, it has always held the speed it was going (61 mph or above) when I got going up the other side. Doesn't matter if it's set in DRCC or Classic Cruise.

 

I would be fine with a cruise control that kept a 5-6 mile per hour window (±2-3 mph) instead of the ±1 mph window that the Prius seems to keep.

If anything, it’s not loose enough for me. I drive on varied terrain in Colorado where trying to maintain an exact speed is way more inefficient than letting the car approximately keep a target speed.

In Volkswagens and Hondas, I’ve seen the same ±1 mph variation, so the Prius behavior seems acceptable for a cruise control system.

 

You might be able to adjust acceleration rate of the cruise control. In practice, the lowest rate might work out as a larger mph window.