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John D


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Speedometer reading wrong/off
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Grab a sammitch & a beer - this is a long one...

Ever since I did the LT1/4L60e conversion (and bought the spendy modified 700r4 tailcone) the dang speedometer has been wrong... always between 5 to 10 mph slow.

When I ordered the tailcone conversion kit, it asked very specific questions about tire size/diameter & axle ratio - to ensure getting the correct drive & driven gears for the speedo. Regardless, it's been wrong from day one.

I've been to a zillion internet "speedometer error & gear selection calculator" sites, about 1/2 dozen tranny shops (swapping driven gears + 1 tooth, - 1 tooth, etc.), and ALL of them come up with me needing the same driven gear - which is WRONG!! banghead

I've gone as far as marking the tire with chalk, driving exactly 1 revolution, marking again, and measuring the "true" circumference of the loaded tire. Then plugging THAT number into the calculators!  banghead Still shows the wrong tooth count (the gear I'm running).

(Why I couldn't wrap my head around this earlier I don't know...cuckoo)

 So I started to think about this... HARD. Throw out the wierd drive/driven tooth numbers and just give the drive gear a value of "10", and the driven gear a "30". The speedometer reads SLOW. This means the driven isn't turning fast enough. For it to turn faster, it needs less teeth (closer to the # on the drive gear). The current driven is a 40 tooth.

From what I've learned, for every driven tooth subtracted, this adds about 3 mph to the reading... so I need around a 37 tooth driven gear.

Part 2:    Now, through research I've discovered GM makes two different driven gear "housings". Each housing will handle a "range" of driven gears - one for 35 to 39 tooth, the other for 40 to 45. (The hole for the gear shaft is more/less off-center just a tad to compensate for the different OD of the gears, and the "slop" taken up in tooth contact depth.) Of course I've got the 40 to 45 housing!!angry

Flashback 2 years ago. Me being the cheap Sunuva that I am... I did a scrounge session at Upullit and found a mechanical speedo tailhousing for a 700r4. The idea was that I would modify it, and make my own combination electronic VSS & mechanical speedo drive - and save the nearly $400.              (It really wouldn't be that hard, IF you have a lathe, IF you can weld aluminum, IF you have...) I shelved that idea, along with the tailcone assembly.

Enter tonight. Take a guess... What the "aich E dbl hockeysticks" is sitting on the shelf??            Yes.            A 700r4 tailcone, WITH a 35 to 40 housing, WITH A GODDANG 37 TOOTH DRIVEN GEAR IN IT!!!

Put the car in the air, spend 3-1/2 MINUTES swapping out the gear & housing, and now have a speedometer that's GPS confirmed to be within 1 mph of correct!doh Been screwing with this off & on for nearly 2 years.

 

 



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John D. - St. Louis Park, MN.

1965 El Camino - LT-1, 4L60e, 4wh discs, SC&C susp.
2013 F-150 Platinum - Twin Turbo 3.5

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Derek69SS


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Glad you got it worked out! For something that should be so simple, getting it right has given me fits as well (until I went to electronic speedo). An incorrect speedo is annoying as heck.

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Derek Kiefer - Mantorville, MN

69 Malibu Pro-Touring stroker LS1-383/T56 - 69 SS396-325/3spd project
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Sleep well John, I know that keep you up at night.

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Kevin

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SShink


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I gave up on the last Chevelle's speedometer... even with the adaptor made by the speedo shop, it either wouldn't work with the adaptor, or was way off.  I'm ready to pop for a cheap GPS that will stay in the car as the speedo.  I'm one of those strange people that actually use the speedo and not just go with the traffic flow.  laughing



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Stan S.-Twin Cities 'South Metro'

1972 Malibu Convertible 2nd time around 

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jim larson


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SShink wrote:

I gave up on the last Chevelle's speedometer... even with the adaptor made by the speedo shop, it either wouldn't work with the adaptor, or was way off.  I'm ready to pop for a cheap GPS that will stay in the car as the speedo.  I'm one of those strange people that actually use the speedo and not just go with the traffic flow.  laughing

 Me too. The way traffic passes me by you would think the speedo is off by about 15mph.  Just checked it the otherday on a 10 miles stretch.  It was off by 0.4 of a mile.

Last time I got a speeding ticket was in the fall of 1967,  2 tickets within 5 days.  In guess what, a 66 chevelle SS.



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Jim L

Lake City
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Yeah, sometimes I think mine if off, but that's mainly on highways. In town it seems right on. Hwy 10 in the A.M. is bumper to bumper doing 80+.

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Bryan-NW 'burbs
1972 Malibu
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1999 std bore 5.7, Vortec heads, Holley Stealth Ram, GM cam
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Chris R


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My speedo never worked since the day I got it. I always just judged my engine RPM's in high gear on the freeway. 3500 RPM was right around mid 60's MPH. The cable was broke at the transmission and I just never got around to replacing it so I just lived with it from 93-06. Stock transmission and gears. I remember reading posts on Team Chevelle by others with the same combination of a 3:31 rear, same M21 trans and the same 60 series 15 inch wheel diameter, were also commenting on thier rpm/speedo readings. So I knew then I was in the ballpark. I use a GPS for work now and was just noticing the other day how dang accurate these things are.

I hear what Derrek is saying. A speedo that is off is annoying to me too. Ive found that to see a speedo that is off much more annoying than if it doesnt work at all.



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Chris - Ramsey, MN.

Dear Optimist, Pessimist, and Realist.

While you guys were busy arguing about the glass of water. I drank it!

Sincerly,

The opportunist.
John D


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Sleep well John, I know that keep you up at night.

"Stuff" that doesn't work just bugs the Censored out of me, and yes, occupies a part of my head called the "nagging problem file". This file has LOTS of folders, and the contents will come forward and chew at me at the most unopportune moments - especially when it's staring you in the face every time you're driving the car. (Fer cryin' out loud the CLOCK and AM radio in my car works (!!), but the speedo's been off for 18 months!)

I'm slowly accepting GPS, but why have one to tell your speed when the bulk of your instrument cluster is occupied by a device specifically there to tell you the speed??

I'm just glad the dang thing works properly now, and THAT job can be dumped from the nag file.



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John D. - St. Louis Park, MN.

1965 El Camino - LT-1, 4L60e, 4wh discs, SC&C susp.
2013 F-150 Platinum - Twin Turbo 3.5

2018 Factory Five MkIV Roadster build thread
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