Speedometer accuracy?

[SANOLS1]

I’m just wondering if anyone else has noticed a deviation with their speedometer versus the Head Up Display and the DIC message when Cruise is set.

Lately I’ve noticed that my speedometer reads about 1.5 mph slower than the HU display. To top it off the other day I was cruising at 74mph according to the Speedo, I set the cruise and the DIC says “Cruise set at 75” and the HUD displays 76Mph. I have a Garmin GPS that agrees 100% with the Speedo so I assume the HUD is the unit in error. Is this a common anomaly or is the General just wanting to err on the side of the consumer to keep the speeding tickets a dollar or two cheaper? :eyerole

 

[c5y2k]

Mine acts the same. I set the cruze at 75 HUD reads 76.

 

[vett boy]

Mine also varies slightly but never more than 1 mph

 

[DoomDriver]

This must be a GM design as mine does exactly the same

 

[o2bnkc]

Mine too.

 

[hoosierdaddy]

Mine too but I assumed the speedometer was closer to be the acurate one but it is never more than 1 mph. And being that neither one of them are calibrated or certified I never thought much of it.

Bill

 

[crzyvette]

Same here..........

 

[Patrick]

I've noticed this too, and I'll tell you where I've noticed it most acutely- at very slow speeds. According to the owner's manual, I have to be at or below 3 MPH before I can shift into reverse. I normally like to park back-end in (in "launch position") in a parking space or my garage. So when I slow down to a stop, although the speedo says I'm stopped, it takes a second or two for the HUD to catch up to that.

I've always chalked that up to the fact that the HUD is a secondary information system, receiving it's information from other sensors (speed, fuel level, tachometer, etc) before it can display them on the glass.


-Patrick

 

[hoosierdaddy]

I've noticed this too, and I'll tell you where I've noticed it most acutely- at very slow speeds. According to the owner's manual, I have to be at or below 3 MPH before I can shift into reverse. I normally like to park back-end in (in "launch position") in a parking space or my garage. So when I slow down to a stop, although the speedo says I'm stopped, it takes a second or two for the HUD to catch up to that.

I've always chalked that up to the fact that the HUD is a secondary information system, receiving it's information from other sensors (speed, fuel level, tachometer, etc) before it can display them on the glass.


-Patrick

That is kind of the way I have looked at it too, it takes a second or two for the HUD to calibrate/calculate the transfer of data from sensors or something. Even though I've notcied this it is not enough to concern me. But once you've used HUD it is hard to drive something that doesn't have it. We have a GP with HUD and it appears to have the same characteristics other than it has more data on the HUDS than I do on the Vette, radio info, temp. and etc.
Bill

 

[SANOLS1]

I've noticed this too, and I'll tell you where I've noticed it most acutely- at very slow speeds. According to the owner's manual, I have to be at or below 3 MPH before I can shift into reverse. I normally like to park back-end in (in "launch position") in a parking space or my garage. So when I slow down to a stop, although the speedo says I'm stopped, it takes a second or two for the HUD to catch up to that.

I've always chalked that up to the fact that the HUD is a secondary information system, receiving it's information from other sensors (speed, fuel level, tachometer, etc) before it can display them on the glass.


-Patrick

I've definitley noticed it coming to a stop on many occasions where the HUD is winding down from 3mph to 0 when I'm already stopped. Your synopsis is probably right on for that instance. I was more concerened about how it compared at (ticket) speed. Suffice to say, if I go by the HUD I'm probably going to be better off if it comes to splitting hairs with a CHiP who goes by the 10mph over rule.

 

[hoosierdaddy]

I've definitley noticed it coming to a stop on many occasions where the HUD is winding down from 3mph to 0 when I'm already stopped. Your synopsis is probably right on for that instance. I was more concerened about how it compared at (ticket) speed. Suffice to say, if I go by the HUD I'm probably going to be better off if it comes to splitting hairs with a CHiP who goes by the 10mph over rule.

I know what you mean, I got the "Silver Bullit" that makes chirps of its own that helps with this. I don't think the HUD is that much off, maybe at the most 1 mph and maybe just a touch less responsive than the speedometer. When you come to a complete stop it takes take a bit for it to drop to 0. I just don't see a significant differance to worry about myself!

Bill

 

[Patrick]

I've definitley noticed it coming to a stop on many occasions where the HUD is winding down from 3mph to 0 when I'm already stopped. Your synopsis is probably right on for that instance. I was more concerened about how it compared at (ticket) speed. Suffice to say, if I go by the HUD I'm probably going to be better off if it comes to splitting hairs with a CHiP who goes by the 10mph over rule.

Bear in mind that you're speedometer is in a constant state of re-calculating to be accurate. If you're crusing along a highway in the country, the variables your speedometer will have to adjust for actual distance over time, tire wear, road conditions (in case your experiencing any tire slippage), etc. Only when it's calculated that can it send the info to the HUD, so there is a slight time delay in getting that updated information.

The difference, if you're crusing along, won't be signficant. The real differences between what speed you are actually going and what speed the HUD shows you travelling will only be off if you are constantly changing your rate of acceleration or deceleration- that's where it's secondary information status has to play catch up. For normal highway cruising, it will be pretty well synched with your speedometer.

-Patrick

 

[hoosierdaddy]

Patrick,

That is about the way I looked at it and the way I have seen it work. If you make a quick stop in town there is a slight delay in the HUD to the speedometer. I assumed the HUD was getting information collected from all of the souces thus the reason for the slight delay. I pretty much rely on it because it is right in front of me but the difference is not significant and the delay does not bother me at all. This is just the way it is and it probably not off as much as the delay makes it appear to be.

Bill