3.90 gears WILL affect your speedometer
Burning Brakes
Joined: Jan 2008
Posts: 905
Likes: 0
From: BURTON MI
Our voices sometimes fall on deaf ears. Gears do not change the speedo on a manual, tire size will, and the new tire size can be entered into the program to correct the speedo reading. JMHO.
Last edited by BOB EAGAN; Sep 19, 2009 at 10:52 PM.
Race Director
Joined: Sep 2003
Posts: 13,550
Likes: 6
From: Now in God's Country, the Big Sky, Montana!
Cruise-In 7-8-9-10 Veteran
St. Jude Donor '06-'07-'08-'09-'10-'11-'12
Safety Car
Joined: Nov 2007
Posts: 4,537
Likes: 1,403
From: Bethesda MD
I know it is not supposed to require adjustment, but in my case it did. The readings on my speedo matched my GPS and gear calculators before the swap. After the swap it was off 3 mph at 60 and 4 mph at 75 mph.
I'm not talking theory, just reporting what happened after swapping to 3.90s. Maybe there is something else going on since the entire rear was replaced and there are other after market parts in it such as the Z06 shafts?
All that is being suggested is that you check your readings before and after a gear swap.
I guess gremlins came and stole my factory wheels and tires and replaced them with an identical looking set that happened to be 5% different. 
I'm not talking theory, just reporting what happened after swapping to 3.90s. Maybe there is something else going on since the entire rear was replaced and there are other after market parts in it such as the Z06 shafts?
All that is being suggested is that you check your readings before and after a gear swap.
Tech Contributor
Joined: Jul 2002
Posts: 14,950
Likes: 264
From: St. Louis, MO
St. Jude Donor '08
Probably worth discussing how the inductor wheel works....Lee???
To my understanding and observation of my blown up rear end (see below) the inductor ring is on the ring gear toward the right, and the sensor is above it, in the hole right where my case cracked in half.
Is the reluctor wheel replaced in a gear change, or is the factory reluctor reused?
Does the gearset offset the reluctor wheel from it's factory place causing the sensor to read inaccurately? It doesn't seem like it to me, unless there is a factory spacer between the carrier and the ring gear that is not used when you put in the 3.90 gearset (which is possible, as the gearset might be thicker or thinner - remember the "3-series" carriers that are valuable when modding a 12 bolt)?
Seems these are good questions to ask. Anyone here care to chime in? I can ask my shop tomorrow I suppose.
To my understanding and observation of my blown up rear end (see below) the inductor ring is on the ring gear toward the right, and the sensor is above it, in the hole right where my case cracked in half.
Is the reluctor wheel replaced in a gear change, or is the factory reluctor reused?
Does the gearset offset the reluctor wheel from it's factory place causing the sensor to read inaccurately? It doesn't seem like it to me, unless there is a factory spacer between the carrier and the ring gear that is not used when you put in the 3.90 gearset (which is possible, as the gearset might be thicker or thinner - remember the "3-series" carriers that are valuable when modding a 12 bolt)?
Seems these are good questions to ask. Anyone here care to chime in? I can ask my shop tomorrow I suppose.
Last edited by Joe_G; Sep 20, 2009 at 11:07 AM.
Thread Starter
Safety Car
Joined: Jan 2006
Posts: 4,598
Likes: 78
From: Miami FL
2020 C6 of the Year Finalist - Unmodified
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15-'16,'17,'18-'19-'20-'21-'22
I know it is not supposed to require adjustment, but in my case it did. The readings on my speedo matched my GPS and gear calculators before the swap. After the swap it was off 3 mph at 60 and 4 mph at 75 mph.
I'm not talking theory, just reporting what happened after swapping to 3.90s.
All that is being suggested is that you check your readings before and after a gear swap.
I'm not talking theory, just reporting what happened after swapping to 3.90s.
All that is being suggested is that you check your readings before and after a gear swap.
Thread Starter
Safety Car
Joined: Jan 2006
Posts: 4,598
Likes: 78
From: Miami FL
2020 C6 of the Year Finalist - Unmodified
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15-'16,'17,'18-'19-'20-'21-'22
Is the reluctor wheel replaced in a gear change, or is the factory reluctor reused? Does the gearset offset the reluctor wheel from it's factory place causing the sensor to read inaccurately? It doesn't seem like it to me, unless there is a factory spacer between the carrier and the ring gear that is not used when you put in the 3.90 gearset ?
Seems these are good questions to ask. Anyone here care to chime in? I can ask my shop tomorrow I suppose.
Seems these are good questions to ask. Anyone here care to chime in? I can ask my shop tomorrow I suppose.
) and Jeremy apparently had never seen this before when he adjusted my tire size.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
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
