Speedometer calibration for changed wheels
Thread Starter
Heel & Toe
Joined: Jan 2007
Posts: 23
Likes: 1
From: Jackson NJ
Speedometer calibration for changed wheels
Put a set of 18/19 Z06 wheels on a non Z06 (old wheels were 17/18's). Have heard that this could cause the speedometer to read incorrectly.
What is the difference in readings (%)??
What is the difference in readings (%)??
Race Director
Joined: Oct 2002
Posts: 16,477
Likes: 26
From: Gone
CI 6-7-8-9-10 Veteran
St. Jude Donor '03
And also wrong, no calibration changes are needed for a gear ratio change. On A4 cars, some transmission shift point changes are necessary, but that's the result of the RPM changing at given speeds, not the gears themselves.
To the OP, upgrading to larger wheels doesn't necessarily mean tire height changes significantly. Lower profile tires are used with larger wheels, this generally keeps the rolling diameter about the same, and little or no effect on speedometer accuracy. I am running 19/20" tires that are exactly the same diameter as the stock 17/18" tires, so no programming changes were needed.
If you post your rear tire size, I can tell you how different from stock they are, and how far off (and which way) they will affect your speedometer. Yes, tire height changes can be changed in the PCM calibration, but realize it only affects speedometer/odometer accuracy (and other tune factors which rely on MPH input), but not other systems like traction control, ABS, etc, that also use the wheel speed sensors to function.
Last edited by Y2Kvert4me; Apr 16, 2010 at 03:50 PM.
Pro
Joined: Dec 2009
Posts: 606
Likes: 1
From: Waco Texas
How do you figure tire diameter does not alter the sensed wheel speed?
And also wrong, no calibration changes are needed for a gear ratio change. On A4 cars, some transmission shift point changes are necessary, but that's the result of the RPM changing at given speeds, not the gears themselves.
To the OP, upgrading to larger wheels doesn't necessarily mean tire height changes significantly. Lower profile tires are used with larger wheels, this generally keeps the rolling diameter about the same, and little or no effect on speedometer accuracy. I am running 19/20" tires that are exactly the same diameter as the stock 17/18" tires, so no programming changes were needed.
If you post your rear tire size, I can tell you how different from stock they are, and how far off (and which way) they will affect your speedometer. Yes, tire height changes can be changed in the PCM calibration, but realize it only affects speedometer/odometer accuracy (and other tune factors which rely on MPH input), but not other systems like traction control, ABS, etc, that also use the wheel speed sensors to function.
And also wrong, no calibration changes are needed for a gear ratio change. On A4 cars, some transmission shift point changes are necessary, but that's the result of the RPM changing at given speeds, not the gears themselves.
To the OP, upgrading to larger wheels doesn't necessarily mean tire height changes significantly. Lower profile tires are used with larger wheels, this generally keeps the rolling diameter about the same, and little or no effect on speedometer accuracy. I am running 19/20" tires that are exactly the same diameter as the stock 17/18" tires, so no programming changes were needed.
If you post your rear tire size, I can tell you how different from stock they are, and how far off (and which way) they will affect your speedometer. Yes, tire height changes can be changed in the PCM calibration, but realize it only affects speedometer/odometer accuracy (and other tune factors which rely on MPH input), but not other systems like traction control, ABS, etc, that also use the wheel speed sensors to function.
A helpful tool ( remembering again) to use when adjusting for tire diameter and speedo accuracy is a GPS. After using the diameter I measured ( again as close as possible) I was still off a few MPH and after playing with the tire diameter in HPTuners corrected it to the exact values of the GPS!
Le Mans Master
Joined: Aug 2006
Posts: 9,987
Likes: 25
From: If opportunity doesn't knock, build a door. South West Florida / Livin' The Dream
St. Jude Donor '12
Here's a web site to help you determine the diameter change of your new wheel and tire combo and the speedometer error with the change
http://www.miata.net/garage/tirecalc.html
http://www.miata.net/garage/tirecalc.html
Thread Starter
Heel & Toe
Joined: Jan 2007
Posts: 23
Likes: 1
From: Jackson NJ
old rear tires 275/40/18, new rears 295/35/19.
Based on the calculator, the speedometer is reading 1.8% slower than actual.
Good to know when "discussing" with the authorities.
Thanks for the help guys!
Based on the calculator, the speedometer is reading 1.8% slower than actual.
Good to know when "discussing" with the authorities.
Thanks for the help guys!
Team Owner
Joined: Apr 2001
Posts: 54,204
Likes: 180
From: S.W. Ohio. . . . . . NRA Life Member
it's the exact opposite. Please delete/edit your post.
for speedo calibration; it's not necessarily the different rims.... it's tire heigth. The factory overall tire heigth is 26.25 inches. If you chose taller rims - but lower profile tires - (where the net 'overall heigth' remains the same)... then the speedo will remain accurate.
Safety Car
Joined: Jul 2005
Posts: 3,572
Likes: 9
From: Jackson Tn
St. Jude Donor '06-'07-'08-'09
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
Instructor
Joined: Jan 2025
Posts: 119
Likes: 6
You are absolutey right and I apologize for my forgetfulness sometimes. I went through this ordeal with my MN6 FRC and I now recall that I did in fact have to play with the tire diameter in the PCM to accurately get my speedo back to normal. Now that I remember, what happened is I went 19/20 during the same time I went 3.90 gears out back and was trying to fix speedo. I went in and punched in the accurate ( as possible) tire diameter and also changed the gear ratio from 3.42 to 3.90 in the PCM. It took me a few times to figure out to leave the gear ratio at 3.42 even though I had 3.90 and just get the tire diameter correct. Most vehicles I tune need both inputs written to the PCM for accuracy with the speedo.
A helpful tool ( remembering again) to use when adjusting for tire diameter and speedo accuracy is a GPS. After using the diameter I measured ( again as close as possible) I was still off a few MPH and after playing with the tire diameter in HPTuners corrected it to the exact values of the GPS!
A helpful tool ( remembering again) to use when adjusting for tire diameter and speedo accuracy is a GPS. After using the diameter I measured ( again as close as possible) I was still off a few MPH and after playing with the tire diameter in HPTuners corrected it to the exact values of the GPS!
Tech Contributor
Joined: Jan 2007
Posts: 19,350
Likes: 1,124
From: Dyer, IN
Instructor
Joined: Jan 2025
Posts: 119
Likes: 6
It doesn't set if speed above 50Mph, but it does if less. Even if I set when it is less than 50, and increase it though the CC button the moment vehcile speed hits above 50 , thr cc gets disengaged
Tech Contributor
Joined: Jan 2007
Posts: 19,350
Likes: 1,124
From: Dyer, IN
Tough to diagnose without a scanner to determine what is causing the disconnect.
