Radar Cruise Control off by 1mph

normal, some are off by 2 mph.

 

I have the same problem. Bisco, why do you say it's "normal". It might be common but not normal.

 

from what i've read here, all cars run slower than the speedo, by legislation. i assume the drcc is based off of that, no?

 

As measured how? By the speedometer, by a GPS, by a roadside radar, by an OBDII-port monitor, or by something else? Expect several different answers among these methods.

 

same here. It always show going on 69, when i set DRCC at 70.

 

It is normal and a kind of safety feature in the software. It will always deviate one or two mph lower when selecting cruise in the radar and laser cruise in Lexus. I have seen it in black in white but that was years ago at our Tech school house in Jacksonville.

 

So speed sensors see the speed signal and input it to the ABS/ Skid Control ECU and relays it via high speed CAN to ECM, COmbo Meter and other systems that will use your speed input for a function in safety.

 

The fix is simple. Always set it to 1 higher than your desired speed...

 

exactly.

 

Welcome to my world as a tech. If, and I have, said that to a customer they would roll their eyes and income the bad survey, lol.

 

It's "normal" on my prime and it was the same with my Gen 3 Prius with DRCC. I think it's just rounding and it tries very hard not to go over what is set.

 

it also depends on if you're on a flat surface or going uphill or downhill

 

My liftback Prius sometimes does this, but I don’t consider it a problem: I assume it reflects a choice by the designers of the cruise control system to tolerate a small offset, under some conditions, in exchange for greater stability, with better resistance to oscillation, hunting, and overshoot. See any textbook on automatic control systems for an explanation of the theory and trade-offs involved.

In simple terms, it’s better for the cruise control to bring the car’s speed very close to the set point, and then leave it there, than to repeatedly speed up and slow down in a futile attempt to chase the exact set point.

There haven’t been U.S. speedometer regulations since 1982. In Japan and Europe, the regulations don’t prohibit accurate speedometers, but if there is an error, the indicated speed must be higher than the actual speed. Manufacturers design their products for world-wide sale accordingly.

The same correction should be applied to both the speedometer and set point displays. The offset is more likely to be related to the control system design, as explained above.

 

The speedometer is set to read high. The radar cruise control seems to use GPS speed, at least in Pearl S. So the result is if the cruise is set for 70 your speedometer will read a bit more. If it's the other way then it's weird but still not out of the question.

And yes, it is "normal". The instruments in a car ARE NOT accurate to 1 MPH in 70. You are lucky if you have 10% or better accuracy.

 

ahhh... this seems completely normal. if you set it to 70 MPH, and the car shows 70, is it rounding DOWN or rounding UP? (e.g. if you're doing 70.1 does the car show 70 or 71?)

so if you set it to 70 mph, I would expect the car to go no more than 70 EXACTLY. more likely 69.9. otherwise you'd be violating a <= condition in the software. sure... they could have done a floor() or ceil() call to do some other rounding, but they probably just did a <=

 

mine is off by 1mph too.

 

I assume this is done on purpose to give the car just a bit of leeway for road conditions, because the one thing the car is good at, is NOT going above what you set the speed to. So if you set your speed to 66mph, the car will normally stay at 65... but I've noticed if you then start going down a hill or a grade, it will sometimes go up to 66mph and stay there. That extra 1mph gives a bit of room for the change in road conditions. Someone figured it's better to to have it go 1mph less than to have it go over what you set it to (after all, say you set it to 65mph and the speed limit is 65mph and upon going down a grade the car surpassed that for a minute at 66mph, you could theoretically get a ticket and Toyota doesn't want to be responsible for it).

 

That seems a bit extreme...If your speedometer was off by 10%, that's +/- 7mph @ 70mph...
I think we're all seeing accuracy a bit better than that.