Analog Speedo / HUD Mismatch?
Thread Starter
Le Mans Master
Joined: Aug 2000
Posts: 9,341
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From: Las Vegas Nevada
Analog Speedo / HUD Mismatch?
I've noticed since I got my '04 a little over a year ago that the Speed displayed on the HUD and the speed displayed on the instrument cluster are rarely ever tyhe same with the cruise control set. Example, if I set the cruise at 70MPH the HUD will normally display 71MPH. It almost always shows 1mph faster on the HUD then what the cruise is set at... It's not anything I'm worried about, it just seems odd to me... Is this normal or is it an indication of something I should be looking into?
Racer
Joined: May 2009
Posts: 279
Likes: 4
From: Cincinnati OH
I notice the same thing and also that the HUD sometimes does not match what the "Cruise set at XX mph" message on the DIC display reads as the setpoint.
While we are on the subject, there's another feature I have noticed and wonder if it's somehow related:
When using the cruise button(s) to increment & decrement speed, the "Cruise set at XX mph" message on the DIC display usually indexes by values of one (i.e. 77, 78, 79, 80, 81, 82) but occasionally indexes in two's. (i.e. 77, 78, 80, 82)?
Anybody?
Thread Starter
Le Mans Master
Joined: Aug 2000
Posts: 9,341
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From: Las Vegas Nevada
Mines does that too. I think its just a difference in the rounding because the throttle isn't set in steps. if its actually set at 80.6 it shows 81 on the HUD. If I'm stepping the cruise up with the button it almost always skips 79 and jumps strait from 78 to 80mph...
Racer
Joined: May 2009
Posts: 279
Likes: 4
From: Cincinnati OH
I did some searching here and found several threads discussing these topics.
As far as the HUD and analog speedometer variance, the answer that makes sense to me is that these two seperate sysetms have a different tolerance rating for accuracy (analog +/-3% and digital HUD +/-1%).
However, I have not found a definative answer explaining the cruise setpoint indexing by twos other than what appears to be speculation.
As far as the HUD and analog speedometer variance, the answer that makes sense to me is that these two seperate sysetms have a different tolerance rating for accuracy (analog +/-3% and digital HUD +/-1%).
However, I have not found a definative answer explaining the cruise setpoint indexing by twos other than what appears to be speculation.
Last edited by lespaulr0cker; Apr 7, 2010 at 01:23 PM.
Thread Starter
Le Mans Master
Joined: Aug 2000
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From: Las Vegas Nevada
Pro
Joined: Apr 2007
Posts: 585
Likes: 2
My '98 coupe is also guilty of both of these things. Typically if the speedometer is showing 70 mph and I set cruise control, the "cruise set at" message will display 68 mph. If I then step the speed up a click on the cruise stalk, the "cruise set at" display jumps by two mph.
I guess it is better that my speedometer reads higher than having it the other way around (as long as the speedometer is the less accurate of the two). That should hopefully help keep me a little more in line with the law.
LO PHAT
I guess it is better that my speedometer reads higher than having it the other way around (as long as the speedometer is the less accurate of the two). That should hopefully help keep me a little more in line with the law.
LO PHAT
Pro
Joined: Apr 2002
Posts: 578
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From: Dallas TX
Mine has been this way since the day I drove it off the dealers lot. I just stopped looking at the speedo and only watch the HUD
As far as the sometimes jumping 2 miles in cruise, mine does that too and I will have to agree the I think it is the computer rounding partial MPH. Maybe actually not rounding but truncating so 68.9 = 68 on setting then when you go to bump it up it sets itself to the next whole number, 70.
Just my theory on the cruise behavior...
-Ed
As far as the sometimes jumping 2 miles in cruise, mine does that too and I will have to agree the I think it is the computer rounding partial MPH. Maybe actually not rounding but truncating so 68.9 = 68 on setting then when you go to bump it up it sets itself to the next whole number, 70.
Just my theory on the cruise behavior...
-Ed
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Tech Contributor
Joined: Jan 2007
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From: Dyer, IN
Like I said in the OP, it's not anything I'm worried about, just curious if its a common thing or something else. If its not something anyone else has noticed it could be an indication of electrical gremlins starting to wear on the HUD. It's not like no C5 has ever had an electrical problem...
Instructor
Joined: Feb 2009
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From: Lockport IL
Team Owner
Joined: May 1999
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From: Columbus Ohio
CI 3-4, 8-9-10 Veteran
St. Jude Donor '06-'08-'10
My analog and HUD vary by 1 or 2 MPH. Add the GPS in and the both read about 2 MPH higher than the GPS.
Not stock wheel sizes on the rear though...
Not stock wheel sizes on the rear though...
Thread Starter
Le Mans Master
Joined: Aug 2000
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From: Las Vegas Nevada
Thats good to know. It just struck me as wierd because I didn't notice it for the first 5-6 months I had the car but its been doing it for the last 6 months so I figured I'd ask
I probably just wasn't paying attention the first 6 months...
I probably just wasn't paying attention the first 6 months...
Drifting
Joined: Mar 2010
Posts: 1,628
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From: Seattle WA
Well okay, anyone else notice the HUD will continue to display 4-3-2-1-0 MPH after you've come to a stop? Takes it a moment or two to actually hit 0.
My guess is that the HUD has a different formula for time averaging. I suspect the speedo needle is more instantaneous, and the HUD reading is averaged over a second or so timeframe.
Either that, or nobody else's car does this and mine's crazy. Wouldn't be the first time I was alone in a room full of people.
My guess is that the HUD has a different formula for time averaging. I suspect the speedo needle is more instantaneous, and the HUD reading is averaged over a second or so timeframe.
Either that, or nobody else's car does this and mine's crazy. Wouldn't be the first time I was alone in a room full of people.
Team Owner
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From: Columbus Ohio
CI 3-4, 8-9-10 Veteran
St. Jude Donor '06-'08-'10
Team Owner
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From: S.W. Ohio. . . . . . NRA Life Member
The DIC display of cruise control "setting" MPH does not round-off. If you set your cruise control while traveling 65.6mph... the DIC shows a setting speed of "60" and the HUD will round-it-up to 61.
As far as speedo variance; the speedometer introduces an analog element. Where the HUD & DIC maintain electronic data integrity throughout. 99% of the time - trust the HUD/DIC over the speedometer.
Racer
Joined: May 2009
Posts: 279
Likes: 4
From: Cincinnati OH
ding... ding... ding; we have a winner !
The DIC display of cruise control "setting" MPH does not round-off. If you set your cruise control while traveling 65.6mph... the DIC shows a setting speed of "60" and the HUD will round-it-up to 61.
As far as speedo variance; the speedometer introduces an analog element. Where the HUD & DIC maintain electronic data integrity throughout. 99% of the time - trust the HUD/DIC over the speedometer.
The DIC display of cruise control "setting" MPH does not round-off. If you set your cruise control while traveling 65.6mph... the DIC shows a setting speed of "60" and the HUD will round-it-up to 61.
As far as speedo variance; the speedometer introduces an analog element. Where the HUD & DIC maintain electronic data integrity throughout. 99% of the time - trust the HUD/DIC over the speedometer.
Team Owner
Joined: Apr 2001
Posts: 54,204
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From: S.W. Ohio. . . . . . NRA Life Member
I'd bet money that the "resolution" for MPH is in the hundredths of a MPH... inside the PCM/BCM; but the readout instruments (speedo - DIC HUD) get a rounded off product.
Instructor
Joined: Jun 2008
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From: Justin TX
Don't know about documentation, but the math tells me it has to be the DIC rounding up (or down). I noticed my vette did the same thing the first week I owned it. I drive 95% of the time on highway, so I use the cruise a lot. I found that if I set the cruise at 65, the HUD would show 65 or 66, so I set the cruise higher, at 90 to see if the variation changed, like it would with a tire size change without recalibrating the speedo. The variation was still only 1 mph. I tried again at 120 with the same results. The math says that at 120, the HUD should have been off twice as much at 120 as is was at 65(I know 60 is half of 120, but close enough) and it wasn't. It has to be rounding to the nearest mph for the computerized displays.