3.90 gears WILL affect your speedometer
Thread Starter
Safety Car
Joined: Jan 2006
Posts: 4,598
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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
3.90 gears WILL affect your speedometer
Two of us have had exactly the same phenomenon after getting 3.90 gears installed, check THIS THREAD for details.
I suspect the 3.90 ring gear is different in some subtle way from the GM 3.42 ring gear such that your speedo will read 5% HIGH (it will say you are going 60 mph when you are really going 57 mph).
You can correct this via tuning by reducing the stated size of your rear tires by 5%. Then your speedo and your GPS will agree with each other again.
I suspect the 3.90 ring gear is different in some subtle way from the GM 3.42 ring gear such that your speedo will read 5% HIGH (it will say you are going 60 mph when you are really going 57 mph).
You can correct this via tuning by reducing the stated size of your rear tires by 5%. Then your speedo and your GPS will agree with each other again.
Race Director
Joined: Mar 2005
Posts: 10,399
Likes: 25
From: Austin Texas
According to GM it shouldn't affect the speedometer. The speedo runs off a reluctor on the ring gear so when you change ring gears you also change to the correct reluctor.
If you are just going by an Internet table then the table will show the speed difference between the 3.42 and the 3.90 gears but since the reluctor is part of the ring gear then there is no speedometer change on a C6.
If you are just going by an Internet table then the table will show the speed difference between the 3.42 and the 3.90 gears but since the reluctor is part of the ring gear then there is no speedometer change on a C6.
Burning Brakes
Joined: Mar 2007
Posts: 1,107
Likes: 9
From: Wichita Falls, TX
According to GM it shouldn't affect the speedometer. The speedo runs off a reluctor on the ring gear so when you change ring gears you also change to the correct reluctor.
If you are just going by an Internet table then the table will show the speed difference between the 3.42 and the 3.90 gears but since the reluctor is part of the ring gear then there is no speedometer change on a C6.
If you are just going by an Internet table then the table will show the speed difference between the 3.42 and the 3.90 gears but since the reluctor is part of the ring gear then there is no speedometer change on a C6.
Race Director
Joined: Oct 2005
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From: Saint Albans, WV
Burning Brakes
Joined: Jan 2008
Posts: 905
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From: BURTON MI
According to GM it shouldn't affect the speedometer. The speedo runs off a reluctor on the ring gear so when you change ring gears you also change to the correct reluctor.
If you are just going by an Internet table then the table will show the speed difference between the 3.42 and the 3.90 gears but since the reluctor is part of the ring gear then there is no speedometer change on a C6.
If you are just going by an Internet table then the table will show the speed difference between the 3.42 and the 3.90 gears but since the reluctor is part of the ring gear then there is no speedometer change on a C6.
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Race Director
Joined: Mar 2005
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From: Austin Texas
From previous posts over a couple of years;
Changing REAR TIRE size will affect the speedometer, changing ring and pinion will not since the speedometer sender (speedometer "gear") is in the ring gear.
Speedometers are not that accurate anyway. Those of us with HUD know there is a difference between the dash speedo and the HUD reading. GPS readings would be interesting, I wonder if the speedo reading is set for a new tire or 1/2 worn tire? Tire size (diameter) changes with wear and the effective speedometer readings change also.
Stock rear diameter is around 26.7", with 8/32" tread the wear alone could reduce that diameter to 26.2".
All of this is pretty much nit picking anyway, who ever got a ticket for 1 or 2 MPH over the speed limit?
Changing REAR TIRE size will affect the speedometer, changing ring and pinion will not since the speedometer sender (speedometer "gear") is in the ring gear.
Speedometers are not that accurate anyway. Those of us with HUD know there is a difference between the dash speedo and the HUD reading. GPS readings would be interesting, I wonder if the speedo reading is set for a new tire or 1/2 worn tire? Tire size (diameter) changes with wear and the effective speedometer readings change also.
Stock rear diameter is around 26.7", with 8/32" tread the wear alone could reduce that diameter to 26.2".
All of this is pretty much nit picking anyway, who ever got a ticket for 1 or 2 MPH over the speed limit?
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
According to GM it shouldn't affect the speedometer. The speedo runs off a reluctor on the ring gear so when you change ring gears you also change to the correct reluctor.
If you are just going by an Internet table then the table will show the speed difference between the 3.42 and the 3.90 gears but since the reluctor is part of the ring gear then there is no speedometer change on a C6.
If you are just going by an Internet table then the table will show the speed difference between the 3.42 and the 3.90 gears but since the reluctor is part of the ring gear then there is no speedometer change on a C6.
What I am suggesting is:
1. If you are changing gear ratios, check the speedo calibration, and adjust it via the tire diameter if required
2. The suppliers of RPL ring gears should check the affect that has on the reluctor and speedo signal, and clean up their acts.
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
Team Owner
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From: salem OR
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
Racer
Joined: Jan 2009
Posts: 405
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From: SPRING CREEK NEVADA
I have oem tires and wheels too. I have a second set of 18,s that do bring the speedo closer to gps as they are alittle smaller dia. the 3.90,s have changed my speedo and it appears others as well.
Melting Slicks
Joined: Feb 2006
Posts: 2,007
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From: Eastern PA:ECS Paxton Novi 1500(676rwhp,585tq on stock engine),LG headers, 410's, Corsa Sports, ECS Alky Kit
St. Jude Donor '08-'09
As others have said the gear change has zero effect on ur speedo reading. It would be the same amount off by GPS now as it was before the gear change.
Pro
Joined: Apr 2008
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From: Paris
Tech Contributor
Joined: Jul 2002
Posts: 14,950
Likes: 264
From: St. Louis, MO
St. Jude Donor '08
I will say FOR SURE that tire diameter affects the speedo. When I run my 24.5" short drag radials, my speedo reads about 4-5 mph high according to the radar trap on 869.
Safety Car
Joined: Jul 2006
Posts: 4,647
Likes: 1
From: Objects in your mirror are losing , Long Island, NY
St. Jude Donor '08
From previous posts over a couple of years;
Changing REAR TIRE size will affect the speedometer, changing ring and pinion will not since the speedometer sender (speedometer "gear") is in the ring gear.
Speedometers are not that accurate anyway. Those of us with HUD know there is a difference between the dash speedo and the HUD reading. GPS readings would be interesting, I wonder if the speedo reading is set for a new tire or 1/2 worn tire? Tire size (diameter) changes with wear and the effective speedometer readings change also.
Stock rear diameter is around 26.7", with 8/32" tread the wear alone could reduce that diameter to 26.2".
All of this is pretty much nit picking anyway, who ever got a ticket for 1 or 2 MPH over the speed limit?
Changing REAR TIRE size will affect the speedometer, changing ring and pinion will not since the speedometer sender (speedometer "gear") is in the ring gear.
Speedometers are not that accurate anyway. Those of us with HUD know there is a difference between the dash speedo and the HUD reading. GPS readings would be interesting, I wonder if the speedo reading is set for a new tire or 1/2 worn tire? Tire size (diameter) changes with wear and the effective speedometer readings change also.
Stock rear diameter is around 26.7", with 8/32" tread the wear alone could reduce that diameter to 26.2".
All of this is pretty much nit picking anyway, who ever got a ticket for 1 or 2 MPH over the speed limit?
Pro
Joined: Apr 2008
Posts: 728
Likes: 30
From: Paris
You are accurate on the speedo versus tach readings. The speedo reads 150rpms higher than the Hud reading with rpms at idle. I haven't checked the the speedo versus rpm driving..HMMM..I can tell you this that my BFG's KDW 2s have 20k on them and still have plenty of thread left for everyday driving. However, the speedo reads 2 miles per hour higher than the GPS reading. It wasn't so when the tires were new. I guess the tire diameter has an adverse effect on the speedo reading with wear.
a++ Cedric