Radar Cruise Control off by 1mph

mine have always been off about 6.5% at 30mph, according to speed radar signs. but it varies with speed. no idea at 70, maybe it improves?

 

mine's off by 2.8% at 35 MPH according to the speed radar sign, if I'm travelling 36 MPH with cruise set at 36 the speed radar sign says 35 MPH

 

I think all of us in North America see better than that. But don't generalize that to European and Asian markets, they are often biased higher.

 

If I'm doing 80kmh I'll see 78~80 on my Garmin

This is kinda off-topic though. I think OP's concern was that if he set the cruise when (for example) 50 mph was displayed, it would subsequently stay at a displayed speed persistantly lower, around 49. Maybe depends if you were coming up on that speed, vs steady cruising?

All-in-all seems a very minor issue, and if you're not happy, one uptick of the cruise wand and you're sorted?

Kinda like that scene in The Jerk, the guy's plane has leather seats that are cracking.

 

as I said... I believe this to be by design. the cruise speed you set is the MAXIMUM. if you were set at 40 and doing 40.1 you would be exceeding the maximum speed. it's almost impossible for it to be 40.0000000 so under almost all circumstances you'll see it at 39.

it's just a < or <= condition in their code. i'm positive this has nothing at all to do with regulations (the drcc bit), but everything to do with differences in rounding between systems

 

Did not read entire thread.
1) All speedos have an error ration.
2) 1 MPH is outstanding accuracy IMO.
3) Tire wear could account for 1 MPH (tire diameter reduces over time).

 

Regardless of what causes it or whether or not it is "normal" or how many other cars do it or don't do it........having the cruise speed end up being 1 MPH different than what you thought you set it to..........is not a PROBLEM.

Just hit the "tap up" button once and enjoy your car.
Or better yet, don't sweat the small stuff.

 

As of Gen 4, the cruise control is "digital" meaning you actually set a number. Thus it is easy to compare set speed to the speedometer.

 

I've seen displayed speed under and over set speed, 3rd gen with dumb cruise.

 

i'm sure there's conditions that will cause it to go over and under. certainly there's debounce code in there to create an underdamped condition otherwise the control loop would be too tight (for instance the heating control in your house... if you have the ac set at 70 it may actually cool it down to 68 in order to minimize the constant cycling).

likewise, if you have the cruise set at 70 and you suddenly hit a high downhill you may exceed the maximum as the software slowly builds break pressure in order to minimize the jerk.

 

I'll see speed fall a bit climbing a hill, then overshoot a bit at the crest. Nature of the beast.