Speedo gear help

bitchin'71SS

Veteran Member
Sep 10, 2009
397
Poughkeepsie, NY
Surprisingly, after using TCI's speedo gear calculator and ordering/installing the gear suggested, my speedometer is still totally off..

So, what driven gear or gear set should I go with? Here's the specs:

TH350
4.10 drive axle ratio
225/70R15 (27.4"d according to TCI calc)
Pink 7-tooth drive gear.

TCIcalc.jpg


I bought the red 21 tooth driven gear. With a 21.1 calculated driven gear, I thought it would only be a few miles off cruising but actually when I'm doing 60 it reads 80, 70 it reads 95-100, 90 it reads 120...

I pulled the cover and double checked the axle ratio, pulled out the speedo cable and double checked the driven gear, and that's definitely my tire size right there.

So, if anyone is running 4.10's with stock tires, what speedo gears do you have? I know roughly how fast I'm going by the rpm's, but I would really like it to be accurate. Thanks guys
 

455 Formula

Veteran Member
Dec 8, 2009
1,417
Fresno, California
The calculators are an estimation in my opinion.

It has been written that each tooth you add to the speedo driven gear will reduce the speedometer reading about 5%.

In my actual experience, with the Muncie (which uses a similar style gear, each tooth will change the speedo reading by about 2 mph.

Your reading of 70 mph at around 90 mph is a 37% error.

I would really have to suspect your speedo head itself could be out of calibration.

Maybe Gauge marks will chime in....Robert
 

77RS

Veteran Member
Lifetime Gold Member
Oct 18, 2006
2,944
Asheboro, NC
455 Formula said:
... I would really have to suspect your speedo head itself could be out of calibration.

Maybe Gauge marks will chime in....Robert

There are a lot of things to factor in when changing tire sizes, gear ratios, etc... The most important thing is that the speedo is accurately calibrated before changing the driven gear. Was it accurate before the differential change?

The speedos in our cars are calibrated for 1000 turns per mile (NOT to be confused with engine RPMs). At 1000 turns of the speedo cable, the speedo should read 60 MPH. The original spec tolerance is +2/-1. A change in the magnetism, distance between the magnet and speed cup, trash in the mechanism, bushing gap, etc can cause problems. Just something to consider.
 

vfw_1168

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Lifetime Gold Member
May 3, 2010
1,209
wheelersburg,ohio
my speedo reads 10 mph off all the time ... i am running a smaller tire as my rims offset is +19 with the stock tire i was rubbing the wheel well with those rims , so if i am doing 60 it reads 70 , with the stock wheels and tires it was 5 mph off 60 was was doing 55 mph ... my tranny is the borg warner and the rear gears is the stock 3.42
 

theflash

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Lifetime Gold Member
Feb 27, 2004
11,044
Podunk, CT
I have a th350 and 4.10 gears and 28" tall tires also and IIRC my speedo gears are a 7/21 combo also. My odometer is dead nuts to my gps, so I assume my speedo is correct too. I would suspect something else might be the problem. Are you 100% sure you have the correct speedo gears by tooth count? Just going by the color is not 100% accurate. Also less likely, but worth asking, are you 100% sure you have 4.10's?
 

Cardinal

Veteran Member
Lifetime Gold Member
Jun 22, 2003
8,354
Endicott, NY
Let's kick this can one more time.

The goal of calibrating a speedometer is to get the drive to driven speedometer gear ratio.

Here's how to get to that end. You need to know two pieces of data:

1) You need to know what the tire size is. Either the diameter or the size (like 2.25/70/15).

2) You have to know the rear end gear ratio.

You then plug that information into a speedometer calculator. I prefer the one at Bowtie Overdrives: http://www.700r4.com/speedoCalc/speedocalc.shtml

Your data: 225/70R15 and 4.10 gears gave a resultant of a 15 drive gear and a 45 driven gear. Divide 45 by 15 = 3.000

Now go to this link that I posted here that has a chart on it to help you find the correct drive and driven gears: http://www.nastyz28.com/forum/showthread.php?t=150376&highlight=speedometer

Scan the chart and you will see that there are quite a few pairs of gears that will give you a 3.00 ratio: 7 & 21, 8 & 24, 9 & 27, 10 & 30, 11 & 33 and so on.

Then you need to acquire the gears. I buy mine from the local GM dealer mainly because I have a "go to" counter person that knows HOW TO LOOK THEM UP!

Your's was an easy calculation. But let's say that the final gear ratio was an odd number like 2.589 . Scan my chart for numbers that are as close as you can to 2.589 . 7 & 18, 8 & 18 , 11 & 25 are just a few on that chart that will be close enought to make the speedometer accurate.

EDIT: I re-read your an everyone elses posts and agree that your speedometer head must be off. I've used the above method to calibrate MANY speedometers and found it to be VERY accurate.
 
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bitchin'71SS

Veteran Member
Sep 10, 2009
397
Poughkeepsie, NY
I didn't really consider it before, but i think you guys are right, the speedometer itself must be out of calibration. I'm 110% sure about the info posted, so by process of elimination.. yeah. It's a 10+ year old auto meter sport comp gauge. I plan on contacting auto meter and getting it re-calibrated. Recently I had them replace a bulb in my tach (non-accessible type) it was only like $30 and they sent me basically a whole new gauge. Hopefully it's not much more than that, I'd rather not have to buy a brand new unit.

Big thanks, some great info just got laid down.. I think this should do it.:happy:

Flash, I'm glad you mentioned that because now I KNOW something ain't right..

e-mailing autometer tomorrow for sure.
 

Gary S

Administrator
Lifetime Gold Member
Apr 14, 1999
24,520
Bismarck, North Dakota
All these cutsie speedometer calculators on computers assume that your speedometer is accurate. That is a big mistake to assume.

The simplest way to get a speedometer right is the oldest way. You don't worry about gears, tire size, or any of that crap the calculators use to screw you up.

All you need to know is how far your existing combination is off and in which direction. Then you use that percentage of error and change gears by that percentage. The old simple way works 100% of the time, even if your speedometer has error in it. The new fangled stupid way of expecting a computer to figure it out for you gets you just what you got...... WRONG.

Do it the old easy way and get it right the first time.
 

77RS

Veteran Member
Lifetime Gold Member
Oct 18, 2006
2,944
Asheboro, NC
Speedometer 101

Hope this will help clear up the understanding of how a speedo operates... :D

A speedometer is calibrated independently of the driven gears, etc. The driven gears (in the tranny) are used to match the driveline with a calibrated speedometer. There must be a constant, so In our cars, the speedo is calibrated for 1000 turns per mile. To figure the MPH at any given number of turns, multiply by .06 (ie: 1000 x .06 = 60 MPH or 842 x .06 = 51 MPH)

The speedometer works off of the magnetic field created by the turning magnet and the speed cup assembly. A small change in the magnetism can cause huge differences in the speedo readings and accuracy.

The odometer works off of gears, so the odometer can be completely accurate while the speedometer is 30 MPH off. I see it all the time.

A perfect example is that an 85 MPH speedo and a 150 MPH speedo is exactly the same unit (except for the '81 85 MPH). The difference is in the magnetic field created by the rotating magnet and the speed cup assembly. I can recalibrate an 85 MPH to be a 150 mph by changing the magnetism and of course, the speedo face. (This takes specialized equipment, so don't try this at home ;) ) All of these speedos use the same odometer.

A MPH speedo can be converted to a KPH speedo by changing the magnetic field. To change the odometer to read kilometers instead of miles, the third worm gear has to be changed. Although a speedo and odometer are in the same housing, their operation is actually independent of each other.

Hope this helps!
Daniel
 
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bitchin'71SS

Veteran Member
Sep 10, 2009
397
Poughkeepsie, NY
Thanks! That was actually really helpful. I had no idea there was that much going on in there.

AND, apparently niether does the guy I just talked to at AutoMeter. He's telling me that the mechanical speedometer is calibrated only by the gears in the trans.. ??? I said no, I think it's a little more complicated than that, he said no it's about as simple a device as can be.. Gave me a # for one of their buddy companies that sold speedo cables and gears and maybe they could help me. :mad: He basically told me there was no calibration necessary on a mech speedo, it just works. So what the hell...

-The speedo I'm using is a mechanical Sport-Comp 3992. I got it used but it's probably around 12 years old.
 




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