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Old 07-02-2006, 04:53 PM
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speedometer gearing way off.

My 41 pontiac with original speedo and Th350 tranny don't get along well. At 45 I'm registering about 34. The drive gear is green (18 tooth) and the driven is Blue (20 tooth) The biggest drive gear listed is 20 tooth (brown) but i haven't found one yet. the smallest driven gear is 18 tooth (Brown). That would up my speedo reading by 23%. that would take me up to about 41 MPH, still 10% slow. Is there any other way to alter the speedo reading?
Is there a supply for gears outside of the OEM specs?

By the way, a buddy has already made the obvious suggestion "just put 13 inch wheels on the back and slow the car down to match the speedo". See why I'm looking for professional help.

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Old 07-02-2006, 05:14 PM
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There are some aftermarket speedo shops that advertise in Street Rodder Magazine that sell speedo calibration gearboxes that plug into the trans to alter the ratio beyond what you can do.

All you have to do is tell them 34 > 45 and they will send you a box that makes it correct.
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Old 07-02-2006, 06:18 PM
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Quote:
Originally Posted by oldguy829
My 41 pontiac with original speedo and Th350 tranny don't get along well.
Is there a supply for gears outside of the OEM specs?
Not be a smart ***** but have you tried your GM dealer ??
I bought some Turbo 400 speedometer gears recently from my Chevrolet dealer. He had to order them ( 2 day wait ) ...
Or go here ... they have what you need ...
http://www.tciauto.com/components/speedometer_gears.htm
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Old 07-03-2006, 02:39 PM
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speedo gears way off

Deuce, that's a good site. It's where I got the information I posted, and my first set of gears. They just don't have the ratio I need.
xcentrix - found a site with a bolt on unit like you described. They have one for a 38% adjustment. I'm going to double check my error, but that looks real close. Thanks.
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Old 07-03-2006, 03:44 PM
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tire diameter x 3.14159= circumference (C)
63360/C= tire rotation per mile (Tr)
Tr x Axle ratio = driveshaft rotation speed (Ds)
divide Ds by 1000= Speedo ratio (Sr)
Use Sr to determine gearing, ie. Sr=2.43
2.43 x 18 = 43.75
This would give you an 18 tooth drive gear and a 44 tooth driven gear.
Plug in the specifics for your car and you can compute your speedo gear ratio. BPO driven gear housings range from 34 to 39 tooth and 40 to 44 tooth.
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Old 07-03-2006, 05:26 PM
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speedo gears way off

Chazmac, I would prefer to do it your way. It is a mid 70's Nova rear end with no markings I can find. How do I determine axel ratio? Also the speedo is not marked. How do I find speedo ratio for a stock 41 pontiac?
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Old 07-03-2006, 05:35 PM
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if speedo is reading slower than actual vehicle speed then it means the cable is not spinnning fast enough and you need to decrease driven gear size or increase drive ring size

gm dealer or trans shop or junkyard and online parts boards, count your teeth and go from there

use the formula if possible too,

stopwatch on freeway going at 60mph from one marker to the next should be 60 seconds, this will tell you if your cable is too fast or too slow
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Old 07-03-2006, 07:31 PM
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Old guy,
GM and most aftermarket speedos are calibrated for 1000 RPM at 60 MPH; this is what my calculations are based on. Rear gear ratio is a little tougher, esp. if you do not open diff cover. If you have a helper, mark the driveshaft or pinion yoke, and have someone help you rotate the tires forward. Turn the tires one revolution and count the number of turns on the pinion yoke. One tire turn to 3 1/4 pinion turns makes a 3.23 diff for most GM diffs. You get the idea. If there is no diff tag, this is the easiest way to get diff ratio without removing the cover.
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Old 07-04-2006, 10:11 AM
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Quote:
Originally Posted by xntrik
There are some aftermarket speedo shops that advertise in Street Rodder Magazine that sell speedo calibration gearboxes that plug into the trans to alter the ratio beyond what you can do.

All you have to do is tell them 34 > 45 and they will send you a box that makes it correct.

I'll tell ya what always works for me. Radar gun or GPS. Just tell the calibration people "reads X at actual 60" and they will do the rest. I prefer the reading to be 1 faster than the actual.

Oh Ya, don't forget that the magnets inside the speedometer could be getting weak. A good speedo shop will determine that the speedo itself is reading 60 @ 1000 rpm cable input. If it doesn't then the odometer will be incorrect.
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Old 07-04-2006, 04:20 PM
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I am always changing tire sizes gear ratios and transmissions.What I have found to work for me is to have a friend drive at 60 mph and pace them for a couple miles to determine were the speedo is at 60mph.Changing the gears generally changes the speedo 2 1/2 mph for each tooth.I just pull the gear and see how many teeth I need to add or subtract then go to the wrecking yard trans core pile and pull one that has the teeth I need and pull the gear off the output shaft to go with it.This has worked for me through all the transmission,tire and rearend gear ratio changes on my 58 and on the 68 f-100 I had and all the vehicles I have swapped to modern drive trains.Good luck with your project

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Old 07-04-2006, 11:17 PM
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Are you sure about the tooth count on the drive gear? The green one should have 9 teeth.

The surest way to get the gear ratio is to open up the rear end and count the teeth, or look on the edge of the ring gear for a series of four 2-digit numbers, such as 41 15 03 70. Those numbers indicate the tooth counts on the gears and the date of manufacture. Those exact numbers, for example, would indicate a 41-tooth ring gear, a 15-tooth pinion gear, a 2.73 ratio (41 divided by 15 = 2.73), and a date of March, 1970.

If it's not a posi-type rear end, you can jack up ONE rear wheel, then count how many times the drive shaft turns when you turn the lifted wheel TWO full turns. That number is the gear ratio.

http://novaresource.org/speedo.htm has gear counts and ratios for speedo gears. I don't know if your speedo is like later ones that have the 60mph/1000rpm calibration, but if so, this chart might help you out.
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Old 07-05-2006, 01:58 PM
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speedo gears way off

Chazmac and jimfulco. Turning the rear tires and counting the revs on the pinion is so simple I never thought of it. Puzzled because one of you said one rev, and the other said 2 revs? It is not a positraction.
xcentrix. Thanks for the tip, hadn't thought about speedo error either. Spun the cable with a drill, speedo moved, end of brain waves.
Will definetly get it checked before I start buying gears.
It is definetly 18 tooth. Counted them and matched color when I replaced it. It's a th350 by the way. i noticed other trans had different counts on same color gears.
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Old 07-05-2006, 04:21 PM
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the green 18 tooth drive gear should be a gear for a BOP style tail housing.... The drive gears for a chevy tail housing & BOP are different diameters .


They usually do not interchange with good results.


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Old 07-05-2006, 06:29 PM
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Quote:
Originally Posted by DOUBLEDICK
I am always changing tire sizes gear ratios and transmissions.What I have found to work for me is to have a friend drive at 60 mph and pace them for a couple miles to determine were the speedo is at 60mph.

ject

What makes you think that his speedo is closer than plus or minus 2-3 mph?
Why go buy the wrong gears?
Radar or GPS is the only accurate way.

Surely you guys know somebody with a GPS that you can borrow.
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Old 07-05-2006, 09:55 PM
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speedo gears way off

Crosly, took this rookie a minute to figure out BOP.
I went to the chart for GM gears. They didn't differentiate between Chevy and Buick/Olds/Pont. anyway, the 18 tooth Green matched what came out, and fit properly. Does that mean it is not a chevy trans?

Back to calculating the rear end ratio guys. If I jack up one wheel and spin it, is it one rev or two, to count the pinion revs?

As for accruacy, our Burb has one of those portable radar trailers that flashes your speed as you approach. They put it out on the streets from time to time as a friendly reminder. I'll ask the chief if it is calibrated accurately.
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