Speedo gear chart vs. math...

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mbaird

mbaird

mbaird
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I am running a 34 tooth speedo gear with 3.55s and a 25" tall tire... the chart says I need a 36 tooth.

My car currently turns 3000 rpm and the speedo reads 40 mph... Wallace calculator says I am doing 63 mph...

I dont see going up 2 teeth correcting the speedo error ?

What am I missing ?
 
Think in percentages.
How much percent fast/slow is the gauge compared to real speed?
Add/substract the percentage of teeth on the gear to correct.
Adding teeth slows the speedo down.

Use a gps speedometer to geet the actual speed, and not something from a book, internet site or slideruler.
 
Use a Gps Speedo app on your phone and get a better idea how fast you are really going. They are free.
 
mbaird said:
I am running a 34 tooth speedo gear with 3.55s and a 25" tall tire... the chart says I need a 36 tooth.

My car currently turns 3000 rpm and the speedo reads 40 mph... Wallace calculator says I am doing 63 mph...

I dont see going up 2 teeth correcting the speedo error ?

What am I missing ?
Click to expand...
More teeth!

Jake
 
A pal had a 27 in her Challenger from way back when it had stock tires & a 7 1/4" rear.

3.23 8 3/4 added, 15" tires too.

Chart said she needed a 33 with her combination so I swapped one in & it is now spot on at 60 MPH indicated & with the GPS too.

I suck at math.
 
percent works perfect as a correction, especially with automatics and TC slip. But I think you are gonna find that the closest you can get to perfect with some tires is about 2.5%.

3000/3.55s/25s does math out pretty close to 63, so you would need a correction of 40/63x 34= 21 teeth..... which is preposterous.
I would assume something is wrong with a) the tach, or B) the TC, or C) the speedo, or D) that 34tooth gear and it's install, or the 3.55s ain't 3.55s

Bottom line is; You are right to suspect that 2 teeth ain't right,lol..Good catch
 
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mbaird said:
I am surprised AJ hasnt responded yet...lol
Click to expand...

lol,
I work Saturdays 'til 3.
Then I run and water my pooch.
Then I have lunch, and check the thermometer on the deck.
Then I take my boots off and have a pee, and procrastinate about doing outsidework.
Then the pooch wants some lovin".
Then I check and answer my E-mails.
Then I watch a U-tube Christian-themed video or two.
Then I gets me a Coke and some chips,
and then finally I check into FABO.
I got a life ya know,
lol.
 
I think the tach is correct.

I hope the Speedo is correct since I paid a member here handsomely to go thru my gauges...but that scenario makes the most sense.

I will toss a 42 tooth in that I have laying around for kicks.
Maybe I have 4.30s in my rear ?.... dont think so...
 
The chart I use says you need a 37 for that tire diameter & rear gear set - if they are 3.55s.

That is easy enough to check.
 
37 is only a 10% change. I am off by 25%.
Should be a straight forward process...urgh...
 
think about it; the more teeth on the pinion, the longer it will take to make one revolution, and so, the slower the speedO will read.

The speed-o needle is driven by magnetism; it has no connection to the cable whatsoever; the cable just drives a little generator. The generator creates the magnetic force to magnetically drag the needle ever higher with increasing rpm. Anything that affects the generator rpm, affects the needle reading............ unless the needles speed-cup is damaged or dragging or it's bearings are loose.

If you knew the correlation between cable rpm and needle reading, you could drive the cable up to speed with an electric drill, and prove the units accuracy. I don't know it; maybe someone here on FABO knows.
 
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AJ/FormS said:
think about it; the more teeth on the pinion, the longer it will take to make one revolution, and so, the slower the speedO will read.

The speed-o needle is driven by magnetism; it has no connection to the cable whatsoever; the cable just drives a little generator. The generator creates the magnetic force to magnetically drag the needle ever higher with increasing rpm. Anything that affects the generator rpm, affects the needle reading............ unless the needles speed-cup is damaged or dragging or it's bearings are loose.

If you knew the correlation between cable rpm and needle reading, you could drive the cable up to speed with an electric drill, and prove the units accuracy. I don't know it; maybe someone here on FABO knows.
Click to expand...

Damn... you are right...duh...I didnt run the mechanics thru my head. Its like gear inches on a bicycle....
It is most likely my speedo...
I will chart the rpms vs speedo reading today and see if it is consistantant.
 
Not much can go wrong with a speedometer.

Metal shavings / rust particles stuck to the magnet can be an issue, but then the needle is jumpy.

It start by checking the rear end ratio to be sure & then changing the speedometer pinion gear if it were mine.
 
AJ/FormS said:
Anything that affects the generator rpm, affects the needle reading
Click to expand...
It starts on the ground at the rear wheels and works forward to the cable, and utimately to the generator.. There is a reduction gear in the diff and another on the mainshaft, in the back of the trans.
Alaskan_TA said:
Not much can go wrong with a speedometer.Metal shavings / rust particles stuck to the magnet can be an issue, but then the needle is jumpy.
It start by checking the rear end ratio to be sure & then changing the speedometer pinion gear if it were mine.
Click to expand...

Checking the rear ratio, and the clocking on the speedo-housing, are pretty easy
 
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I have had these gears in and out many times and I am sure they are 3.55s. I have verified the clocking of speedo gear in trany.

Here are the readouts for rpm vs speedo reading.

2000 rpm = 25 mph
3000 rpm = 40 mph
4000 rpm = 50 mph

The speedo action is smooth and steady..
 
mbaird said:
I have had these gears in and out many times and I am sure they are 3.55s. I have verified the clocking of speedo gear in trany.

Here are the readouts for rpm vs speedo reading.

2000 rpm = 25 mph
3000 rpm = 40 mph
4000 rpm = 50 mph

The speedo action is smooth and steady..
Click to expand...
Stable but not linear
From 2000 to 3000 is 1000 rpm and 15 mph.
From 2000 to 4000 is also 1000 rpm, but now only 10 mph. 33% less..... something does not compute

Now here's the stumper ;the speedo is supposed to be linear.
At 3000rpm you should be going 63 mph. This is 21mph per 1000 rpm. So at 1000 it should read 21, at 2000 it should read 42 and at 3000 it should read 63mph.

Now looking at your install, at 1000 rpm it would theoretically be reading 12.5mph so 12.5 mph per 1000. Therefore at 3000 it should be reading 37.5 and it's pretty close.

But working the 4000/50 reading , backwards, that's also 12.5mph per 1000; so 3000 should be 37.5 and 2000 should be 25
So my guess is you just rounded the 3000 figure up to 40 from that calculated 37.5. So if that is true, then your speedo head looks to be working correctly , allbeit too slowly.
It needs a boost from 12.5 mph/1000rpm to 21mph per 1000 rpm. This is a difference of 60%. Therefore if you have a 34 tooth in there, you would need a .60x34=20.4, gee where have we seen that number before? Lemmee guess, was it in post #10,lol; why yes I think it was.
Which is still preposterous.
Preposterous, because I've never seen anything like that in an A-body of your vintage.

But the math doesn't lie so get yourself a 20/21 and see what happens........ I guess.

You know what; I did see that once, when a guy installed a metric convertor on the cable. Guess what the ratio was Hyup .62 or 62% But it was the other way 62mph is about 100 KPH so that's .62 in reverse or 1/.62=plus 61%
Ima thinking you should check every inch of your cable, looking for that multiplier, starting right at the trans. It's a little rectangular cube about 2.25 x 1.25x 1.125 IIRC, with the cable coming out of one side. and the other may be directly attached at the trans end or somewhere up the cable.
 
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