Speedometere error
Thread Starter
Heel & Toe
Joined: Oct 2006
Posts: 16
Likes: 0
From: Granbury Texas
Speedometere error
Just bought 92 Convertible, last owner said he replaced wider rear tires with stock. Speedometer is about ten miles per hour slow. Anyone have any ideas? Other tires could have not as tall? Does the transmission still have a gear for adjusting tires size/rear end ratio? Thanks thudplt@alltel.net Havent checked the odometer against hiway test track yet.
Elite Torch Red Member
Joined: Sep 1999
Posts: 57,805
Likes: 23
From: Exit 89 GSP,Lakewood,NJ The Land Of Mojo
Cental/South NJ Events Coordinator
CI 1-2-3-4-5-6-7-8-9-10-11-12 Vet
CI-II Burnout & Drag Champ
St. Jude Donor '03-'04-'05-'06-'07
It doesn't matter how wide, it matters how tall.
Stock size for '92 was 275/40R17, if 315/35R17 were put on with teh proper 17 x 11 rim size, there shouldn't be a problem.
If the wider tire was put on the stock 17 x 9.5, it may skew the speedometer readings, but I don't think by 10 mph.
You'll have to find out if a gear swap was ever done, that will throw it off if the speedo gears weren't changed.
here are the part #'s and stuff related to stock speedo gears. Most won't apply if you went with gear ratios other than stock.
http://www.corvetteforum.net/c4/vett...Gearshtm.shtml
If you have aftermarket rims, it's very easy to not match up the proper tire diameter with stock.
Stock size for '92 was 275/40R17, if 315/35R17 were put on with teh proper 17 x 11 rim size, there shouldn't be a problem.
If the wider tire was put on the stock 17 x 9.5, it may skew the speedometer readings, but I don't think by 10 mph.
You'll have to find out if a gear swap was ever done, that will throw it off if the speedo gears weren't changed.
here are the part #'s and stuff related to stock speedo gears. Most won't apply if you went with gear ratios other than stock.
http://www.corvetteforum.net/c4/vett...Gearshtm.shtml
If you have aftermarket rims, it's very easy to not match up the proper tire diameter with stock.
Last edited by Mr Mojo; Oct 16, 2006 at 10:08 AM.
Le Mans Master
Joined: May 2005
Posts: 5,214
Likes: 65
From: Queensland, AUSTRALIA
No i was thinking the same myself Mojo, 10mph too slow @ 60mph would probably be more than just the wrong sized tires id think. But then we dont know what speed its 10mph too slow at.
We also dont know what evidence indicates this discrepensy (i hope its not going to be "the factory tach indicates X,XXXrpm so speed should be XXmph")
When i changed the ratio in my 94 i went from 2.59:1 up to 3.33:1 and 'at a guess', 140kph was about 100-110kph. That is as much as a 25mph (at the absolute most) difference at about 60mph.
We also dont know what evidence indicates this discrepensy (i hope its not going to be "the factory tach indicates X,XXXrpm so speed should be XXmph")
When i changed the ratio in my 94 i went from 2.59:1 up to 3.33:1 and 'at a guess', 140kph was about 100-110kph. That is as much as a 25mph (at the absolute most) difference at about 60mph.
Elite Torch Red Member
Joined: Sep 1999
Posts: 57,805
Likes: 23
From: Exit 89 GSP,Lakewood,NJ The Land Of Mojo
Cental/South NJ Events Coordinator
CI 1-2-3-4-5-6-7-8-9-10-11-12 Vet
CI-II Burnout & Drag Champ
St. Jude Donor '03-'04-'05-'06-'07
Yeah, we can't rely on the factory tach for anything but a general idea.
Now we have to determine how he found the discrepency. did you clock yourself against another car? Was it a speedlimit device that flashes your speed as you approach? Or did you run a measured mile and time it?
This and many more questions on the next episode of "How the speedometer turns"
Now we have to determine how he found the discrepency. did you clock yourself against another car? Was it a speedlimit device that flashes your speed as you approach? Or did you run a measured mile and time it?
This and many more questions on the next episode of "How the speedometer turns"
Thread Starter
Heel & Toe
Joined: Oct 2006
Posts: 16
Likes: 0
From: Granbury Texas
Soeedometer error
Havent driven the car much 50-75 milels. Tires are stock, felt the speedo was slow as I was with traffic on I-20 I wasindicating 50-55 MPH. Caught up with wife in Lexus SC400, asked her speed, I was on bumper, she said 60, I was indicating 50, maybe a mph or so more. I havent checked against hiway markers yet. Dont know any history on the car, last owner said he took off wider tires, not shorter, didnt even seem to know difference. He didnt own car long, 2-3 years, but drove very little. I was just wondering if speedo itself is noted for being in error. I have had 6 C-4z and never noticed anything different. Thanks
Elite Torch Red Member
Joined: Sep 1999
Posts: 57,805
Likes: 23
From: Exit 89 GSP,Lakewood,NJ The Land Of Mojo
Cental/South NJ Events Coordinator
CI 1-2-3-4-5-6-7-8-9-10-11-12 Vet
CI-II Burnout & Drag Champ
St. Jude Donor '03-'04-'05-'06-'07
The speedo in my '95 six speed is dead on, but the tach is off by about 200.
The speedo in my '93 automatic is off by a little, about 2 mph, but I did a gear swap to 3.45's, but put the speedo gears for a 3.54 in. I didn't think it would make much difference, but according to my GPS and some of those speed indicator machines, it shows me being off.
The speedo in my '93 automatic is off by a little, about 2 mph, but I did a gear swap to 3.45's, but put the speedo gears for a 3.54 in. I didn't think it would make much difference, but according to my GPS and some of those speed indicator machines, it shows me being off.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
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
Thread Starter
Heel & Toe
Joined: Oct 2006
Posts: 16
Likes: 0
From: Granbury Texas
Where is the VIN and available options posted for C4
Where are the vins and available options posted for C4. I have auto matic with I think speedo error, I keep seeing something about speed sensor? Is there something between speedo and transmission that could be causing and erroneous speedo reading? I remember going to a web site that had the vin number breakdown where you could tell what options your car has, like axle ratios power passenger seat, sport seats and such, Thanks
Safety Car
Joined: Aug 2004
Posts: 4,908
Likes: 735
From: Southwest Kansas
2022 Corvette of the Year Finalist -- Unmodified
2021 C6 of the Year Winner - Unmodified
RPO codes might be under your console lid (that's where mine are on my 94) Rear ratio should be given as a "G" code (mine has 2 G92, performance axle ratio and a G44 3.07 ratio)
Le Mans Master
Joined: May 2005
Posts: 5,214
Likes: 65
From: Queensland, AUSTRALIA
As teebee1994 said it should be under the center console lid on a 92. The VIN will be on the lower edge of the driver side windscreen, as well as on the end of the driver door, and will also be on the Parts ID label on the center console lid (if its still there)
Thread Starter
Heel & Toe
Joined: Oct 2006
Posts: 16
Likes: 0
From: Granbury Texas
Where to find axle ratios on computer?
What I meant in last query is where on the computer, or forum do I find the statistics section where things like performance data, axel ratios, how many silver, purple, red, white, admiral blue Vettes were made in any one particular year? Thanks
Safety Car
Joined: Aug 2004
Posts: 4,908
Likes: 735
From: Southwest Kansas
2022 Corvette of the Year Finalist -- Unmodified
2021 C6 of the Year Winner - Unmodified
CorvetteActionCenter.com has all that info. I bought the book, Cor-VETTE specs 1984-1996 by Michael Antonick. It has all that info plus more.
here's a link to the 1992 specs http://www.corvetteactioncenter.com/...992/index.html
And here's one to the RPO codes. http://www.corvetteactioncenter.com/...92/92prod.html
here's a link to the 1992 specs http://www.corvetteactioncenter.com/...992/index.html
And here's one to the RPO codes. http://www.corvetteactioncenter.com/...92/92prod.html
Last edited by teebee; Oct 17, 2006 at 09:10 AM.