Speedo is off...fix options?
Thread Starter
Pro
Joined: Oct 2003
Posts: 641
Likes: 2
From: Storm lake Iowa
Speedo is off...fix options?
Just had my speedometer rebuilt & calibrated, and took it out for a test drive Saturday with a friend and a GPS unit...turns out I'm reading 9%-10% too fast at any given speed. I could pull the dash (AGAIN...ouch!) and send it back for recalibration, but I'd rather be driving right now (well, as soon as the control arm is re-bolted and the alignment re-done that is...).
I havent ever looked inside the access panel on my M21 where the speedo cable hooks in...is there an exchangable gear there? If I could get and install a gear with 10% fewer teeth than my current one, my speedometer should then read right on, right?
Anyone know if this is doable and where I'd get the parts?
-Roy
[Modified by Grinchia, 5:30 PM 4/5/2004]
I havent ever looked inside the access panel on my M21 where the speedo cable hooks in...is there an exchangable gear there? If I could get and install a gear with 10% fewer teeth than my current one, my speedometer should then read right on, right?
Anyone know if this is doable and where I'd get the parts?
-Roy
[Modified by Grinchia, 5:30 PM 4/5/2004]
Team Owner
Joined: Aug 1999
Posts: 42,487
Likes: 47
SPARTAN
CI 3-4-5-6-8-9-10 Vet
CI-9 AutoX Winner
CI-3 Go Kart Champ
St. Jude '03-'04-'05-'06-'07-'08-'09-'10-'11
Re: Speedo is off...fix options? (Grinchia)
Do you have a "measured mile" or a road of a known distance that you can check the ODOMETER against? (I guess GPS could do that too). If the speedo AND the Odo are off the same amount, you have a speedometer gear in the transmission problem. If they don't match each other, you have a speedometer calibration issue.
The tranny gear is pretty simple to replace. A different gear with a different number fo teeth will change the output reading. Zip carries a full selection of gears.
The tranny gear is pretty simple to replace. A different gear with a different number fo teeth will change the output reading. Zip carries a full selection of gears.
Thread Starter
Pro
Joined: Oct 2003
Posts: 641
Likes: 2
From: Storm lake Iowa
Re: Speedo is off...fix options? (ddecart)
[QUOTE]Do you have a "measured mile" or a road of a known distance that you can check the ODOMETER against? (I guess GPS could do that too). If the speedo AND the Odo are off the same amount, you have a speedometer gear in the transmission problem. If they don't match each other, you have a speedometer calibration issue.
I hadn't thought of that. Next trip out I'll do a GPS mile and see how closely it agrees with the odometer.
I don't know if the rear end has been changed or not, nor do I even know what it was supposed to be in the first place. Somewhere awhile back someone did a post about rpms at particular speeds in particular gears to infer which rearend you have, I suppose I could look it up. I know I have an M21 close ratio transmission, and my current wheels are very close to stock height.
I hadn't thought of that. Next trip out I'll do a GPS mile and see how closely it agrees with the odometer.
I don't know if the rear end has been changed or not, nor do I even know what it was supposed to be in the first place. Somewhere awhile back someone did a post about rpms at particular speeds in particular gears to infer which rearend you have, I suppose I could look it up. I know I have an M21 close ratio transmission, and my current wheels are very close to stock height.
Race Director
Joined: Mar 2001
Posts: 11,322
Likes: 1
From: Living in the Hartland
Re: Speedo is off...fix options? (Grinchia)
You will need to pull the trans drive gear out and see what one it is then get one 2 sizes diff with more teeth, the more teeth the slower it turns, believe that each step in gear is equal to about 4.5 to 5 mph. they are based on color but you can count the teeth to find out which one you have.
[Modified by Fevre, 2:16 PM 4/6/2004]
[Modified by Fevre, 2:16 PM 4/6/2004]
Racer
Joined: Apr 1999
Posts: 418
Likes: 2
From: Fort Mohave Az.
Re: Speedo is off...fix options? (Grinchia)
Do not change the gear. That would change the "proportional speed" and the difference would vary with your speed. If you are off the same difference at all speeds then the needle was put back on "shifted" that amount.
Steve
Steve
Race Director
Joined: Mar 2001
Posts: 11,322
Likes: 1
From: Living in the Hartland
Re: Speedo is off...fix options? (69L68)
It would have to read 10 mph in park in order to be off 10 mph all the time I would think. Worth checking out though, check it at 40 and 80, if the same amount off then pointer is off, if double off then gear.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
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
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
Team Owner
Joined: Aug 1999
Posts: 42,487
Likes: 47
SPARTAN
CI 3-4-5-6-8-9-10 Vet
CI-9 AutoX Winner
CI-3 Go Kart Champ
St. Jude '03-'04-'05-'06-'07-'08-'09-'10-'11
Re: Speedo is off...fix options? (Fevre)
The odometer check will help nail down what's wrong. Checking speedo error at a couple different speeds (30 & 60) will help, too.
Thread Starter
Pro
Joined: Oct 2003
Posts: 641
Likes: 2
From: Storm lake Iowa
Re: Speedo is off...fix options? (ddecart)
I'll try to clarify:
at 40mph, it reads about 44mph
at 80 mph it reads about 88 mph
...and so on. It's pretty consistently about 10% off at all speeds. My initial thought was that the pre-load on the speedo spring is not quite high enough, and therefore it allows the needle to move a little further than it should for any given speed. If my odometer is dead on accurate, then no doubt this is the case. If however I find that the odometer is 10% ahead of reality, then it must be a transmission gear, and that's where I need to make the swap.
As I said earlier, I don't know if the rear end is correct and haven't tried to calculate it yet. I do know that the speedometer I just had rebuilt was not original...it had the clip-on cable attatchment at the back rather than the screw on that I'm told was correct for '68. Personally I prefer clip on, it's MUCH easier to manipulate in the tight space behind the dash, so when I ordered my new guts, I stuck with the '69 setup. The new one is supposed to be accurately calibrated to stock setup, so WHO REALLY KNOWS!
I'll check the odometer this weekend and update the thread.
-Roy
at 40mph, it reads about 44mph
at 80 mph it reads about 88 mph
...and so on. It's pretty consistently about 10% off at all speeds. My initial thought was that the pre-load on the speedo spring is not quite high enough, and therefore it allows the needle to move a little further than it should for any given speed. If my odometer is dead on accurate, then no doubt this is the case. If however I find that the odometer is 10% ahead of reality, then it must be a transmission gear, and that's where I need to make the swap.
As I said earlier, I don't know if the rear end is correct and haven't tried to calculate it yet. I do know that the speedometer I just had rebuilt was not original...it had the clip-on cable attatchment at the back rather than the screw on that I'm told was correct for '68. Personally I prefer clip on, it's MUCH easier to manipulate in the tight space behind the dash, so when I ordered my new guts, I stuck with the '69 setup. The new one is supposed to be accurately calibrated to stock setup, so WHO REALLY KNOWS!
I'll check the odometer this weekend and update the thread.
-Roy
Team Owner
Joined: Aug 1999
Posts: 42,487
Likes: 47
SPARTAN
CI 3-4-5-6-8-9-10 Vet
CI-9 AutoX Winner
CI-3 Go Kart Champ
St. Jude '03-'04-'05-'06-'07-'08-'09-'10-'11
Re: Speedo is off...fix options? (Grinchia)
If it's a speedo calibration and not the gear, it's hard to say. What I CAN tell you is that you don't want to be poking around in there with a screwdriver that's even remotely magnetized!! :nonod:
Racer
Joined: Apr 1999
Posts: 418
Likes: 2
From: Fort Mohave Az.
Re: Speedo is off...fix options? (Grinchia)
If its 10% off then it would be gears ("range change"). If it were 10 mph off it would be the needle ("zero shift").
Steve
[Modified by 69L68, 7:10 AM 4/7/2004]
Steve
[Modified by 69L68, 7:10 AM 4/7/2004]
Team Owner
Joined: Aug 1999
Posts: 30,995
Likes: 99
From: Ontario
Re: Speedo is off...fix options? (Grinchia)
Yes there is a gear. And they are available for about $5.00 at your local GM dealer.
This site gives useful info re which gear.
http://members.core.com/~faldrich/speedo.htm
[Modified by paul67, 10:16 AM 4/7/2004]
This site gives useful info re which gear.
http://members.core.com/~faldrich/speedo.htm
[Modified by paul67, 10:16 AM 4/7/2004]
Thread Starter
Pro
Joined: Oct 2003
Posts: 641
Likes: 2
From: Storm lake Iowa
Re: Speedo is off...fix options? (paul67)
Thanks a million! If I can fix this for $5.00 and not have to remove the speedometer again, I will be as delighted as the firefly who backed into the fan! :D :D
-Roy
-Roy
Team Owner
Joined: Aug 1999
Posts: 42,487
Likes: 47
SPARTAN
CI 3-4-5-6-8-9-10 Vet
CI-9 AutoX Winner
CI-3 Go Kart Champ
St. Jude '03-'04-'05-'06-'07-'08-'09-'10-'11
Re: Speedo is off...fix options? (Grinchia)
Just do the odometer check to make sure it's off by the same percentage before you change the gear.
Team Owner
Joined: Aug 1999
Posts: 30,995
Likes: 99
From: Ontario
Re: Speedo is off...fix options? (Grinchia)
My 1967 was reading 70mph against a true 60mph as measured by a friend with a C5 (100kph or 62mph). I went from 20 teeth to 22 teeth and things now read true.
Melting Slicks
Joined: Apr 2001
Posts: 3,399
Likes: 278
From: Olmsted Falls, Ohio
Re: Speedo is off...fix options? (Grinchia)
And don't forget that the speedo and gears were calibrated for the factory diameter tires. If you have gone to a different diameter tire then you need to take that into consideration also.
Team Owner
Joined: Aug 1999
Posts: 42,487
Likes: 47
SPARTAN
CI 3-4-5-6-8-9-10 Vet
CI-9 AutoX Winner
CI-3 Go Kart Champ
St. Jude '03-'04-'05-'06-'07-'08-'09-'10-'11
Re: Speedo is off...fix options? (Grinchia)
Where did you get it calibrated? I really need to send mine out. My odo is right on, but my speedo reads well under 1/2 what it should :eek: