Speedometer Inaccuracy

[sytyguy]

Had a buddy ride behind me today and report the correct speed from his truck (unmodified 2005 Silverado). My speedo reads as follows (my reading @ correct reading):

55 @ 52
60 @ 55
65 @ 59
70 @ 62
75 @ 65
80 @ 68

Haven't changed anything since 1999 (white face gauges from AutoSyndicate - the kind you had to remove the needle for), but I can't verify if the speedo was off like this then or if it's gotten worse with time. Odd thing is that it seems to get farther off the faster you go past 60 mph (like the weight of the needle is working against a worn out motor). FWIW, I have stock tires wheels and no changes to any of the circuitry or mechanicals (OEM VSS, DRAC, and stock Road Speed Constant value in bin).

When I reset the needle (after the gauge face install), I set the cruise @ 60 mph (verified by scantool) and put the needle on 60. I can understand if I put the needle slightly higher than 60 mph (incorrect VSS reporting, etc....), but why wouldn't the inaccuracy be uniform (off the same amount from 0-120)?

Does this seem like a fixable thing (reset the needle)?
Or should I be shopping for new guts for the speedometer?

TIA

Hood

 

[bdydrpds10]

Re: Speedometer Inaccuracy

Mine ty has about 100,xxx miles on it and is all stock. When I got a ticket recently the cop said I was going 7 mph faster than my gauge. So I recently had a buddy
follow me in his lighting and at 55 mph he said his speedo read 60mph wtf:dunno:
Mine seems to be the opposite to yours.

 

[Don W.]

Re: Speedometer Inaccuracy

Well the first thing I'd do is get a stop watch and do some times runs over a measured mile to confirm what you're seeing. Then call around for a good spedo shop and have them check things. They might be able to skip the first step by putting the truck on rollers (4 wheel drive may be a prob.) and they should be able to send a externally generated signal to the spedo to check it. Absent 4 wheel rollers they might be able to plug a counter into the sig generator on the transfer case and take you to a nearby measured mile and do a count. (From the manual it's 4000 pulses/mile. Speed is not a factor here, just to total counts.)

Seems like any error would be linear though so I'm as confused as you.

 

[Ian Turgeon]

Re: Speedometer Inaccuracy

are you able to verify what the gauge is saying vs what the computer is seeing?

 

[Ian Turgeon]

Re: Speedometer Inaccuracy

Well the first thing I'd do is get a stop watch and do some times runs over a measured mile to confirm what you're seeing. Then call around for a good spedo shop and have them check things. They might be able to skip the first step by putting the truck on rollers (4 wheel drive may be a prob.) and they should be able to send a externally generated signal to the spedo to check it. Absent 4 wheel rollers they might be able to plug a counter into the sig generator on the transfer case and take you to a nearby measured mile and do a count. (From the manual it's 4000 pulses/mile. Speed is not a factor here, just to total counts.)

Seems like any error would be linear though so I'm as confused as you.

btw we are appearently 2000 ppm

 

[Don W.]

Re: Speedometer Inaccuracy

btw we are appearently 2000 ppm

??2000?? My info came from the Syclone Supplement. If a Ty is different, my apologies.

 

[It's just a six]

Re: Speedometer Inaccuracy

Hand held GPS works good to show MPH... if you have one?

 

[sytyguy]

Re: Speedometer Inaccuracy

btw we are appearently 2000 ppm

Correctomundo....binary shows 1977-1978 pu/mi.

Are you able to verify what the gauge is saying vs what the computer is seeing?

I'm gonna ride to work tomorrow with DM and double check.

Thanks all.

Hood

 

[sytyguy]

Re: Speedometer Inaccuracy

Hand held GPS works good to show MPH... if you have one?

I do, but it seems to vary a bit (depending on signal strength) so I was gonna trust it as a last resort since I have so many other methods of verifying correct speed.

I can definitely tell ya'll that the speedo is indeed off. Hold 80 mph (read speed) on the interstate and you'll get passed by a polar bear on a unicycle. Besides, just general POV tells me that it's going slower.

Hood

 

[sytyguy]

Re: Speedometer Inaccuracy

Seems like any error would be linear though so I'm as confused as you.

Exactly, this is what has me stumped. A uniform inaccuracy could be remedied by needle adjustment or mental compensation, but I'm not gonna tape a TI-85 to the windshield just to calculate correct speed.

Hood

 

[Don W.]

Re: Speedometer Inaccuracy

Which manual Don? Don't have my GM one here in front of me, but the P4 doc doesn't list road speed info.

I am looking at the 1991 Syclone Service Manual Supplement "page" 8C-1. The speed sensor reads prop shaft speed and sends an analog signal to the DRAC (digital ratio adapter controller) which, in turn, now sends a digital signal to the spedo at 4000 pulses per mile.

HOWEVER: Same book on page 6E3-A-39 (code 24 vehicle speed sensor) it mentions 2000 pulses per mile so now I'm really confused. I suppose it's one or the other.:squint: Another error in the manual??

Anyway I spent a little time looking through the man. and couldn't find the location of the DRAC but from what I could see it looks like its in the cab somewhere and not at the transfer case, just the speed sensor is down there.

 

[JSM]

Re: Speedometer Inaccuracy

Not sure why it is off, tires maybe?

Anyways if you need to correct it let me know. I know how and a chip won't do it.

 

[sytyguy]

Re: Speedometer Inaccuracy

Verified today that the speedo is indeed off versus the VSS as it was in the road test above. Problem appears to be isolated to the speedo assembly iteself.

I am looking at the 1991 Syclone Service Manual Supplement "page" 8C-1. The speed sensor reads prop shaft speed and sends an analog signal to the DRAC (digital ratio adapter controller) which, in turn, now sends a digital signal to the spedo at 4000 pulses per mile.

HOWEVER: Same book on page 6E3-A-39 (code 24 vehicle speed sensor) it mentions 2000 pulses per mile so now I'm really confused. I suppose it's one or the other.:squint: Another error in the manual??

Anyway I spent a little time looking through the man. and couldn't find the location of the DRAC but from what I could see it looks like its in the cab somewhere and not at the transfer case, just the speed sensor is down there.

Gotta be an error. The chip has nearly 2000 ppm as a value for road speed constant, so regardless of what the manual might say, that's what the truck is seeing. DRAC is the white box on the back of the ECM bracket.....under dash, pass side.

Not sure why it is off, tires maybe? Anyways if you need to correct it let me know. I know how and a chip won't do it.

245/50R-16 tires.

PM sent Jeff. Thanks mang!

Hood

 

[Captain Morgan]

Re: Speedometer Inaccuracy

wow, mine is WAY more accurate than that, like within 1 MPH, at least according to those road side radar ticket thingies and one of my friends who is a cop. I know its accurate at 35, 45 and 55 MPH within 1 MPH.

 

[Flyin Ryan]

Re: Speedometer Inaccuracy

Just delete it hood... that way it can't be inaccurate plus it'll net you some more weight savings :devil:

 

[sytyguy]

Re: Speedometer Inaccuracy

Just delete it hood... that way it can't be inaccurate plus it'll net you some more weight savings :devil:

Not a bad idea.....hmmmm. I could use the space for a speaker. Or a shift light. Or to dock my KB 10 sec chip. Or put a mini Price-Is-Right spinoff wheel in there.

Ahhhh, the possibilities.:lol:

Hood

 

[Ian Turgeon]

Re: Speedometer Inaccuracy

Not a bad idea.....hmmmm. I could use the space for a speaker. Or a shift light. Or to dock my KB 10 sec chip. Or put a mini Price-Is-Right spinoff wheel in there.

Ahhhh, the possibilities.:lol:

Hood

hmm, and acceleration g-force based plinko game in the bed sounds cool...