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Topic: Speedo off 5MPH at 60

in Forum: C3 Driveline Components


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Speedo off 5MPH at 60 (1/9)
 6/21/08 3:48pm
sulm68
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Jacksonville, FL - USA

Vette(s):
1968 convertable 327 300hp All numbers matching, rally red with black interior. A/C, power windows, power brakes. Tremic 5speed


Joined: 8/28/2007
Posts: 44

I always suspected my speedometer was incorrect and today I confirmed it. While my speedo read 60 MPH a buddy riding along side confirmed that I was really going 65 MPH. I have 27" dia tires (225R7015 slightly worn). I believe my rear end is a 3.08 and I have a neutral color gear in the muncie 4 speed. Does anyone have any idea what could be going on?
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Speedo off 5MPH at 60 (2/9)
 6/21/08 8:56pm
kstyerLifetime Member
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CUYAHOGA FALLS, OH - USA

Vette(s):
1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!


Joined: 12/2/2003
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The speedometer could be a bit out of calibration.   This is very common on older cars, even when they were new.  The head can be recalibrated.  You may also be able to change the speedo drive gear in the trans.  But the correct gear I will defer to Joel (Adams Apple).  He has those codes/specs.
Speedo off 5MPH at 60 (3/9)
 6/21/08 9:10pm
Brown78
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Ft.Lauderdale, FL - USA

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78 coupe, Dk Brown, ZZ4 385 hp fast burn, steeroid rack/pinion, Keisler 5 speed. Built to run with C5 and C6. on the highway. Can't beat them. But they can't lose me in there mirrors. LOL I built it to enjoy. So far I've tripped through 15 States.


Joined: 6/21/2008
Posts: 31

I found out my speedo was off with my GPS Navigator. It's pritty much on the money at 85 mph. But at 110 mph I'm really only doing 105 mph LOL  I guess I'm OK.
Speedo off 5MPH at 60 (4/9)
 6/21/08 9:50pm
Adams' AppleLifetime Member
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Duncanville, TX - USA

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#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas


Joined: 11/8/2003
Posts: 20126

5mph off can be fixed with a gear change. Take the speedo housing/bullet out of the trans, and count the teeth on the driven gear there. Then get a gear that has one less tooth on it to speed up the reading at the speedo.
I do have a speed-o gear tooth count chart posted here somewhere....I'll look for it. It will be just as easy to count your gear, and then change it to what you need, as what it has may be what it is supposed to have to start with, but the tire size difference is throwing the speed-o off a bit.


                                                         Speed-o Gear ChartAdams' Apple2008-06-21 18:52:36

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Speedo off 5MPH at 60 (5/9)
 6/21/08 10:07pm
sundew
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Eagan, MN - USA

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Gone, but not forgotten: 1974 Coupe Red, matching numbers, Black Interior, L-48 w/ 4 speed; The “Admiral”, also gone and never to be forgotten. 1994 Admiral Blue LT-1 coupe, Black Interior Automatic


Joined: 6/18/2007
Posts: 427

3.08 rear end, 27 inch dia tires, 2300 rpm in 4th =  60 mph. My speedometer is off also. I created a little correction chart that I keep in the car...cheaper and easier than replacing the speedometer drive gear.

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Speedo off 5MPH at 60 (6/9)
 6/21/08 10:32pm
KeBo
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Kendall Park, NJ - USA

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1981 - 59 Beige over Cinnebar leather. Edlebrock top end, 2200 stall w/shift kit, 3:55s, Mufflex maniback exhaust, nice stereo, 16 in AREs w/BFGs


Joined: 3/9/2005
Posts: 789

I've never owned a pre-84 car that was on the money..  My 81 is 5mph optimistic..
Speedo off 5MPH at 60 (7/9)
 6/22/08 5:47pm
sulm68
Former Member

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Jacksonville, FL - USA

Vette(s):
1968 convertable 327 300hp All numbers matching, rally red with black interior. A/C, power windows, power brakes. Tremic 5speed


Joined: 8/28/2007
Posts: 44

Does anyone know how I can confirm my rear end gear ratio?
Speedo off 5MPH at 60 (8/9)
 6/22/08 5:52pm
Ron 78Lifetime Member
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BINGHAMTON, NY - USA

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Joined: 7/19/2003
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I have the same problem,once I get up to "Warp Speed" my speedo ain't accurate at all Rolling%20On%20The%20Floor%20Laughing

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Speedo off 5MPH at 60 (9/9)
 6/22/08 10:37pm
kstyerLifetime Member
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CUYAHOGA FALLS, OH - USA

Vette(s):
1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!


Joined: 12/2/2003
Posts: 6424

IF all else fails, pick up both rear wheels, put the car in neutral.  Mark the drive shaft and the wheels.   Turn the wheels exactly one full turn, and count the number of turns at the drive shaft.  This is your rear end ratio.
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