C5 Speedometer Calibration
Thread Starter
8th Gear
Joined: Jun 2005
Posts: 8
Likes: 0
From: Virginia Beach Virginia
C5 Speedometer Calibration
I have been scouring the Internet for days searching for the answer to this question to no avail:
Is there any ‘do-it-yourself’ way to perform a Speedometer Calibration on a C5 Corvette without having to purchase a $500+ ‘Scan and Tune’ software package or a $300+ handheld unit (like a Predator, etc.)? From what I have found so far, it looks like the key is modifying the Tire Height and/or rear end gear ratio in the PCM. Any of you ‘Scan and Tune’ guru’s out there found a more cost effective/easy way of doing this? Thanks in advance. . .
Is there any ‘do-it-yourself’ way to perform a Speedometer Calibration on a C5 Corvette without having to purchase a $500+ ‘Scan and Tune’ software package or a $300+ handheld unit (like a Predator, etc.)? From what I have found so far, it looks like the key is modifying the Tire Height and/or rear end gear ratio in the PCM. Any of you ‘Scan and Tune’ guru’s out there found a more cost effective/easy way of doing this? Thanks in advance. . .
Racer
Joined: May 2004
Posts: 377
Likes: 3
From: Marietta GA
Cant think of any way. If you have someone near you that has an HPTuner and has a license for that year, ask him to change it for you and take it our for a few beers.
HPTuners dont license down to VIN, just model year.
Dixit
HPTuners dont license down to VIN, just model year.
Dixit
Pro
Joined: Dec 2000
Posts: 747
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From: Columbia SC
If you ever get down to SC I'll take car of it for $75. I'll probably be in Richmond the last week of Sept and could help you out. Email me at: dtsngit@bellsouth.net if you want to discuss.
Mike
Mike
Team Owner
Joined: Apr 2001
Posts: 54,204
Likes: 180
From: S.W. Ohio. . . . . . NRA Life Member
Originally Posted by Since 53
From what I have found so far, it looks like the key is modifying the Tire Height and/or rear end gear ratio in the PCM.
I think the GM Tech-Tool 2 allows this. If so; then my local Chevy dealers charge $75 for a reprog.
Also, there's the forum vendors "$99 mail order tune"; but you would need to send them your PCM (your C5 will not be driveable during the process)
Thread Starter
8th Gear
Joined: Jun 2005
Posts: 8
Likes: 0
From: Virginia Beach Virginia
C5 Speedometer Calibration (continued)
First of all, I'd like to thank Dixit, Mike04, DTSNGIT, and Mike Mercury for your expeditious replies.
In regard to Mike Mercury's reply however I need some clarification on how this thing works. I have no first hand knowledge in this area, so I must give you the benefit of the doubt that what you are saying is correct. I have been told that the circuitry that sends the pulses to the PCM that is eventually translated to the analog voltage which moves the needle on the speedometer; is in the rear end. If that much is true, then why doesn’t changing the rear end gear ratio not change the speed of the electromagnetic pick-ups in the differential (unless of course if the pick-ups are actually tied to the rear axles some how). Please share additional details if you know. Thanks. . .
In regard to Mike Mercury's reply however I need some clarification on how this thing works. I have no first hand knowledge in this area, so I must give you the benefit of the doubt that what you are saying is correct. I have been told that the circuitry that sends the pulses to the PCM that is eventually translated to the analog voltage which moves the needle on the speedometer; is in the rear end. If that much is true, then why doesn’t changing the rear end gear ratio not change the speed of the electromagnetic pick-ups in the differential (unless of course if the pick-ups are actually tied to the rear axles some how). Please share additional details if you know. Thanks. . .
Melting Slicks
Joined: Feb 2002
Posts: 2,613
Likes: 0
From: Winston Salem NC
The Speedometer is driven off the wheel sensor, so it is independent
of any gear change, but will change with tire size.
The Transmission is driven from the VSS, and uses a reluctor on the
ring gear.
of any gear change, but will change with tire size.
The Transmission is driven from the VSS, and uses a reluctor on the
ring gear.
Thread Starter
8th Gear
Joined: Jun 2005
Posts: 8
Likes: 0
From: Virginia Beach Virginia
C5 Speedometer Calibration (continued)
Thanks for the info 'gojo'! Pardon my ignorance (haven't been a C5 owner for that long) but what exactly is a "VSS" and a "reluctor"? Again I apologize, but I'm a Computer Geek not a Gear Head. 
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Melting Slicks
Joined: Feb 2002
Posts: 2,613
Likes: 0
From: Winston Salem NC
Originally Posted by Since 53
Thanks for the info 'gojo'! Pardon my ignorance (haven't been a C5 owner for that long) but what exactly is a "VSS" and a "reluctor"? Again I apologize, but I'm a Computer Geek not a Gear Head.
The relucter is a sensor attached to the ring gear that, as it rotates, sends pulses to the PCM (computer). Those pulses are used to determine "speed" and cause the trans to shift.
This is why when gears only are changed, in a C5, the speedometer will be accurate but the trans shift points will have changed.
