Speedometer Calibration How Many Mph Does One Speedo Gear Tooth Make
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Speedo gear help
Thread starter bitchin'71SS
Commencement appointment
#1
Surprisingly, after using TCI's speedo gear reckoner and ordering/installing the gear suggested, my speedometer is still totally off..
Then, what driven gear or gear ready should I become with? Here'southward the specs:
TH350 4.ten drive beam ratio 225/70R15 (27.4"d according to TCI calc) Pink seven-tooth drive gear.
I bought the cherry 21 tooth driven gear. With a 21.1 calculated driven gear, I idea it would only exist a few miles off cruising simply actually when I'm doing lx 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 at that place.
So, if anyone is running 4.10's with stock tires, what speedo gears do you take? I know roughly how fast I'm going by the rpm's, merely I would really like it to be accurate. Thanks guys
#2
The calculators are an estimation in my opinion.
It has been written that each tooth y'all add to the speedo driven gear will reduce the speedometer reading nearly 5%.
In my bodily feel, with the Muncie (which uses a similar mode gear, each tooth will modify the speedo reading by about 2 mph.
Your reading of lxx mph at around xc mph is a 37% error.
I would really have to doubtable your speedo head itself could be out of calibration.
Maybe Gauge marks will chime in....Robert
#3
... I would actually accept to suspect your speedo head itself could exist out of calibration.
Peradventure Guess marks will chime in....Robert
In that location are a lot of things to cistron 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 information technology accurate before the differential change?
The speedos in our cars are calibrated for grand turns per mile (Not to exist confused with engine RPMs). At 1000 turns of the speedo cable, the speedo should read threescore MPH. The original spec tolerance is +2/-ane. A change in the magnetism, distance betwixt the magnet and speed cup, trash in the mechanism, bushing gap, etc tin can crusade problems. Simply something to consider.
#4
my speedo reads x mph off all the fourth dimension ... 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 , then if i am doing sixty it reads 70 , with the stock wheels and tires it was 5 mph off lx was was doing 55 mph ... my tranny is the borg warner and the rear gears is the stock 3.42
#5
I have a th350 and 4.10 gears and 28" alpine tires also and IIRC my speedo gears are a 7/21 combo besides. My odometer is dead nuts to my gps, so I assume my speedo is correct as well. I would doubtable something else might exist the problem. Are you 100% sure you lot have the correct speedo gears past tooth count? Just going by the color is not 100% accurate. Also less likely, only worth asking, are you 100% sure y'all have iv.10's?
#6
Allow'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. Y'all need to know two pieces of data:
ane) You demand to know what the tire size is. Either the diameter or the size (similar 2.25/seventy/15).
2) You take to know the rear end gear ratio.
You and so plug that data into a speedometer reckoner. I prefer the one at Bowtie Overdrives: http://www.700r4.com/speedoCalc/speedocalc.shtml
Your information: 225/70R15 and four.ten gears gave a resultant of a 15 drive gear and a 45 driven gear. Separate 45 by xv = iii.000
At present become to this link that I posted here that has a chart on it to assistance you lot find the right drive and driven gears: http://www.nastyz28.com/forum/showthread.php?t=150376&highlight=speedometer
Scan the nautical chart and you will see that there are quite a few pairs of gears that will give you lot a 3.00 ratio: 7 & 21, 8 & 24, 9 & 27, 10 & 30, 11 & 33 and so on.
And so you demand to acquire the gears. I purchase mine from the local GM dealer mainly because I accept a "go to" counter person that knows HOW TO LOOK THEM Up!
Your's was an easy calculation. Just allow'due south say that the final gear ratio was an odd number like 2.589 . Browse my chart for numbers that are as close as y'all 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 authentic.
EDIT: I re-read your an everyone elses posts and agree that your speedometer head must be off. I've used the to a higher place method to calibrate MANY speedometers and establish information technology to be VERY accurate.
#vii
I didn't actually consider information technology before, just i recall you guys are right, the speedometer itself must be out of scale. I'm 110% certain near the info posted, then by process of elimination.. yeah. It's a 10+ yr former auto meter sport comp gauge. I plan on contacting auto meter and getting it re-calibrated. Recently I had them supplant a bulb in my tach (not-accessible type) information technology was only like $30 and they sent me basically a whole new estimate. Hopefully it's not much more than that, I'd rather not have to purchase a brand new unit.
Large thanks, some not bad info but got laid downward.. I retrieve this should do information technology.
Wink, I'm glad you mentioned that because now I KNOW something ain't right..
e-mailing autometer tomorrow for sure.
#eight
All these cutsie speedometer calculators on computers assume that your speedometer is authentic. That is a large mistake to assume.
The simplest way to become a speedometer right is the oldest way. You lot don't worry virtually gears, tire size, or any of that crap the calculators use to screw you upwardly.
All you need to know is how far your existing combination is off and in which direction. Then yous employ that percentage of fault and change gears past that percent. The old unproblematic way works 100% of the time, even if your speedometer has error in it. The new fangled stupid style of expecting a computer to figure it out for you gets yous just what you got...... Wrong.
Do it the old piece of cake style and get information technology right the first fourth dimension.
#9
Speedometer 101
Hope this will assistance clear up the understanding of how a speedo operates...
A speedometer is calibrated independently of the driven gears, etc. The driven gears (in the tranny) are used to friction match the driveline with a calibrated speedometer. There must be a constant, and so In our cars, the speedo is calibrated for k turns per mile. To figure the MPH at whatsoever given number of turns, multiply by .06 (ie: thousand x .06 = 60 MPH or 842 ten .06 = 51 MPH)
The speedometer works off of the magnetic field created by the turning magnet and the speed cup assembly. A modest change in the magnetism can cause huge differences in the speedo readings and accuracy.
The odometer works off of gears, so the odometer can exist completely accurate while the speedometer is xxx 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 aforementioned unit of measurement (except for the '81 85 MPH). The divergence is in the magnetic field created by the rotating magnet and the speed cup assembly. I can recalibrate an 85 MPH to exist a 150 mph by changing the magnetism and of form, the speedo confront. (This takes specialized equipment, so don't try this at home ) All of these speedos use the same odometer.
A MPH speedo can exist converted to a KPH speedo by changing the magnetic field. To change the odometer to read kilometers instead of miles, the tertiary worm gear has to exist inverse. Although a speedo and odometer are in the aforementioned housing, their operation is actually independent of each other.
Promise this helps! Daniel
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#10
Thanks! That was actually really helpful. I had no idea there was that much going on in there.
AND, plain niether does the guy I merely talked to at AutoMeter. He's telling me that the mechanical speedometer is calibrated simply by the gears in the trans.. ??? I said no, I retrieve 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 i of their buddy companies that sold speedo cables and gears and possibly they could help me. He basically told me there was no calibration necessary on a mech speedo, it but works. And so what the hell...
-The speedo I'k using is a mechanical Sport-Comp 3992. I got information technology used only it's probably around 12 years sometime.
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